Fix OpenCode session cache source paths

[ozymandiashh]

Summary

Fixes OpenCode session-cache keying by discovering one source per real opencode*.db file instead of one synthetic <dbPath>:<sessionId> source per root session.

The parser now opens the database source, iterates unarchived root sessions inside it, and keeps the recursive child-session parsing added in #343.

Root cause

OpenCode discovery used paths like:

/path/to/opencode.db:root-session-id

The shared session cache fingerprints source paths with stat(). That synthetic path is not a real file, so OpenCode cache entries could be treated as changed or stale on every run.

What changed

• OpenCode discovery now emits path: dbPath for each opencode*.db.
• The parser iterates unarchived root sessions from the DB source.
• Root parsing still walks unarchived child/grandchild session subtrees.
• Legacy compound <dbPath>:<sessionId> source paths are still accepted.
• Old synthetic cache keys are pruned on the next parse because discovery now reports only the real DB path.
• Docs and changelog describe the new cache behavior.

Validation

• Differential synthetic repro against origin/main vs this branch.
• npx vitest run tests/providers/opencode-cache.test.ts tests/providers/opencode.test.ts tests/provider-registry.test.ts - 54/54 tests passed.
• npx tsc --noEmit --pretty false - passed.
• npx vitest run - 876/876 tests passed.
• npm run build - passed.
• git diff --check origin/main...HEAD - passed.
• Argus-style strict review - PASS.
• Claude Opus 4.7 effort max review - PASS.
• Gemini 3.1 Pro Preview review - PASS.

Cache proof

Synthetic OpenCode DB fixture:

root-a
└─ child-a
root-b

Same fixture parsed on origin/main and this branch.

origin/main cache keys:

/tmp/.../opencode.db:root-a
/tmp/.../opencode.db:root-b

This branch cache keys:

/tmp/.../opencode.db

Behavior stayed equivalent:

calls: 3
root sessions: 2
projects:
  tmp-project-a: 2 calls, 1 session
  tmp-project-b: 1 call, 1 session
child dedup key preserved:
  opencode:child-a:msg-child-a-assistant
warm unchanged run rewrote cache: false
legacy <dbPath>:<sessionId> keys pruned: true

Scope and privacy notes

Validation uses synthetic SQLite fixtures only. No real local OpenCode history, prompts, paths, session IDs, usage values, or private project data are included.