Add an encyclopedia of common terms

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+# Encyclopedia
+
+This is a living glossary for T3 Code. It explains what common terms mean in this codebase.
+
+## Table of contents
+
+- [Project and workspace](#project-and-workspace)
+- [Thread timeline](#thread-timeline)
+- [Orchestration](#orchestration)
+- [Provider runtime](#provider-runtime)
+- [Checkpointing](#checkpointing)
+
+## Concepts
+
+### Project and workspace
+
+#### Project
+
+The top-level workspace record in the app. In [the orchestration contracts][1], a project has a `workspaceRoot`, a title, and one or more threads. See [workspace-layout.md][2].
+
+#### Workspace root
+
+The root filesystem path for a project. In [the orchestration model][1], it is the base directory for branches and optional worktrees. See [workspace-layout.md][2].
+
+#### Worktree
+
+A Git worktree used as an isolated workspace for a thread. If a thread has a `worktreePath` in [the contracts][1], it runs there instead of in the main working tree. Git operations live in [GitCore.ts][3].
+
+### Thread timeline
+
+#### Thread
+
+The main durable unit of conversation and workspace history. In [the orchestration contracts][1], a thread holds messages, activities, checkpoints, and session-related state. See [projector.ts][4].
+
+#### Turn
+
+A single user-to-assistant work cycle inside a thread. It starts with user input and ends when follow-up work like checkpointing settles. See [the contracts][1], [ProviderRuntimeIngestion.ts][5], and [CheckpointReactor.ts][6].
+
+#### Activity
+
+A user-visible log item attached to a thread. In [the contracts][1], activities cover important non-message events like approvals, tool actions, and failures. They are projected into thread state in [projector.ts][4].
+
+### Orchestration
+
+Orchestration is the server-side domain layer that turns runtime activity into stable app state. The main entry point is [OrchestrationEngine.ts][7], with core logic in [decider.ts][8] and [projector.ts][4].
+
+#### Aggregate
+
+The domain object a command or event belongs to. In [the contracts][1], that is usually `project` or `thread`. See [decider.ts][8].
+
+#### Command
+
+A typed request to change domain state. In [the contracts][1], commands are validated in [commandInvariants.ts][9] and turned into events by [decider.ts][8].
+Examples include `thread.create`, `thread.turn.start`, and `thread.checkpoint.revert`.
+
+#### Domain Event
+
+A persisted fact that something already happened. In [the contracts][1], events are the source of truth, and [projector.ts][4] shows how they are applied.
+Examples include `thread.created`, `thread.message-sent`, and `thread.turn-diff-completed`.
+
+#### Decider
+
+The pure orchestration logic that turns commands plus current state into events. The core implementation is in [decider.ts][8], with preconditions in [commandInvariants.ts][9].
+
+#### Projection
+
+A read-optimized view derived from events. See [projector.ts][4], [ProjectionPipeline.ts][11], and [ProjectionSnapshotQuery.ts][10].
+
+#### Projector
+
+The logic that applies domain events to the read model or projection tables. See [projector.ts][4] and [ProjectionPipeline.ts][11].
+
+#### Read model
+
+The current materialized view of orchestration state. In [the contracts][1], it holds projects, threads, messages, activities, checkpoints, and session state. See [ProjectionSnapshotQuery.ts][10] and [OrchestrationEngine.ts][7].
+
+#### Reactor
+
+A side-effecting service that handles follow-up work after events or runtime signals. Examples include [CheckpointReactor.ts][6], [ProviderCommandReactor.ts][12], and [ProviderRuntimeIngestion.ts][5].
+
+#### Receipt
+
+A lightweight typed runtime signal emitted when an async milestone completes. See [RuntimeReceiptBus.ts][13].
+Examples include `checkpoint.baseline.captured`, `checkpoint.diff.finalized`, and `turn.processing.quiesced`, which are emitted by flows such as [CheckpointReactor.ts][6].
+
+#### Quiesced
+
+"Quiesced" means a turn has gone quiet and stable. In [the receipt schema][13], it means the follow-up work has settled, including work in [CheckpointReactor.ts][6].
+
+### Provider runtime
+
+The live backend agent implementation and its event stream. The main service is [ProviderService.ts][14], the adapter contract is [ProviderAdapter.ts][15], and the overview is in [provider-architecture.md][16].
+
+#### Provider
+
+The backend agent runtime that actually performs work. See [ProviderService.ts][14], [ProviderAdapter.ts][15], and [CodexAdapter.ts][17].
+
+#### Session
+
+The live provider-backed runtime attached to a thread. Session shape is in [the orchestration contracts][1], and lifecycle is managed in [ProviderService.ts][14].
+
+#### Runtime mode
+
+The safety/access mode for a thread or session. In [the contracts][1], the main values are `approval-required` and `full-access`. See [runtime-modes.md][18].
+
+#### Interaction mode
+
+The agent interaction style for a thread. In [the contracts][1], the main values are `default` and `plan`. See [runtime-modes.md][18].
+
+#### Assistant delivery mode
+
+Controls how assistant text reaches the thread timeline. In [the contracts][1], `streaming` updates incrementally and `buffered` delivers a completed result. See [ProviderService.ts][14].
+
+#### Snapshot
+
+A point-in-time view of state. The word is used in multiple layers, including orchestration, provider, and checkpointing. See [ProjectionSnapshotQuery.ts][10], [ProviderAdapter.ts][15], and [CheckpointStore.ts][19].
+
+### Checkpointing
+
+Checkpointing captures workspace state over time so the app can diff turns and restore earlier points. The main pieces are [CheckpointStore.ts][19], [CheckpointDiffQuery.ts][20], and [CheckpointReactor.ts][6].
+
+#### Checkpoint
+
+A saved snapshot of a thread workspace at a particular turn. In practice it is a hidden Git ref in [CheckpointStore.ts][19] plus a projected summary from [ProjectionCheckpoints.ts][21]. Capture and lifecycle work happen in [CheckpointReactor.ts][6].
+
+#### Checkpoint ref
+
+The durable identifier for a filesystem checkpoint, stored as a Git ref. It is typed in [the contracts][1], constructed in [Utils.ts][22], and used by [CheckpointStore.ts][19].
+
+#### Checkpoint baseline
+
+The starting checkpoint for diffing a thread timeline. This flow is surfaced through [RuntimeReceiptBus.ts][13], coordinated in [CheckpointReactor.ts][6], and supported by [Utils.ts][22].
+
+#### Checkpoint diff
+
+The patch difference between two checkpoints. Query logic lives in [CheckpointDiffQuery.ts][20], diff parsing lives in [Diffs.ts][23], and finalization is coordinated by [CheckpointReactor.ts][6].
+
+#### Turn diff
+
+The file patch and changed-file summary for one turn. It is usually computed in [CheckpointDiffQuery.ts][20], represented in [the contracts][1], and recorded into thread state by [projector.ts][4].
+
+## Practical Shortcuts
+
+- If you see `requested`, think "intent recorded".
+- If you see `completed`, think "result applied".
+- If you see `receipt`, think "async milestone signal".
+- If you see `checkpoint`, think "workspace snapshot for diff/restore".
+- If you see `quiesced`, think "all relevant follow-up work has gone idle".
+
+## Related Docs
+
+- [architecture.md][24]
+- [provider-architecture.md][16]
+- [runtime-modes.md][18]
+- [workspace-layout.md][2]
+
+[1]: ../packages/contracts/src/orchestration.ts
+[2]: ./workspace-layout.md
+[3]: ../apps/server/src/git/Layers/GitCore.ts
+[4]: ../apps/server/src/orchestration/projector.ts
+[5]: ../apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts
+[6]: ../apps/server/src/orchestration/Layers/CheckpointReactor.ts
+[7]: ../apps/server/src/orchestration/Layers/OrchestrationEngine.ts
+[8]: ../apps/server/src/orchestration/decider.ts
+[9]: ../apps/server/src/orchestration/commandInvariants.ts
+[10]: ../apps/server/src/orchestration/Layers/ProjectionSnapshotQuery.ts
+[11]: ../apps/server/src/orchestration/Layers/ProjectionPipeline.ts
+[12]: ../apps/server/src/orchestration/Layers/ProviderCommandReactor.ts
+[13]: ../apps/server/src/orchestration/Services/RuntimeReceiptBus.ts
+[14]: ../apps/server/src/provider/Layers/ProviderService.ts
+[15]: ../apps/server/src/provider/Services/ProviderAdapter.ts
+[16]: ./provider-architecture.md
+[17]: ../apps/server/src/provider/Layers/CodexAdapter.ts
+[18]: ./runtime-modes.md
+[19]: ../apps/server/src/checkpointing/Services/CheckpointStore.ts
+[20]: ../apps/server/src/checkpointing/Services/CheckpointDiffQuery.ts
+[21]: ../apps/server/src/persistence/Services/ProjectionCheckpoints.ts
+[22]: ../apps/server/src/checkpointing/Utils.ts
+[23]: ../apps/server/src/checkpointing/Diffs.ts
+[24]: ./architecture.md