feat: time-travel via epoch markers (git mind at)

[flyingrobots]

Summary

The README claims git checkout syncs the knowledge graph to that commit's point in time. This is false — the CRDT graph lives in refs/warp/gitmind/* and doesn't move with checkout.

Solution

Implement epoch markers — epoch:<sha8> nodes stored in the CRDT graph that correlate git commits to Lamport ticks. git mind at <ref> resolves ref → SHA → epoch → tick → graph.materialize({ ceiling: tick }).

Changes

• New src/epoch.js module (getCurrentTick, recordEpoch, lookupEpoch, lookupNearestEpoch, getEpochForRef)
• src/hooks.js — processCommit records epoch after directive processing
• src/cli/commands.js + bin/git-mind.js — new at command
• src/cli/format.js — formatAtStatus formatter
• src/validators.js — add epoch to SYSTEM_PREFIXES
• src/export.js — exclude epoch nodes from export
• src/doctor.js — exclude epoch nodes from orphan detection
• src/index.js — export epoch functions
• README.md + GUIDE.md — fix false time-travel claims, document git mind at
• New test/epoch.test.js

Acceptance Criteria

• git mind at HEAD shows current graph state with epoch metadata
• git mind at HEAD~N materializes graph at that point in time
• Epoch nodes excluded from export and doctor
• All tests pass

[flyingrobots]

Closed: Epoch markers implemented in src/epoch.js with git mind at CLI command for time-travel.