feat(plugin): session-recap injection — personalized onset greeting

[jeonghun-jj-lee]

Summary

Implements #523 — injects a ## Recent sessions (last 7 days) block into the system prompt so the onset router can personalize its greeting from actual session history, rather than depending on vault files only populated by dream-cycle workflows.

What changed

File	Role
opencode-plugin/session_recap.ts	Core module: DB read (bun:sqlite), filtering, caching, mechanical recap extraction, markdown composition
opencode-plugin/amicode_context.ts	Hook wiring: now async, passes sessionID from input, calls buildRecentSessionsBlock
test/session_recap.test.ts	26 unit tests covering all pure logic paths

Design pivot from issue

The issue assumed client.sessions.list() via a PluginInput SDK handle — investigation showed the plugin receives no client. Pivoted to reading the SQLite DB directly via bun:sqlite (Bun built-in, zero deps). Same outcome, simpler surface.

Deferred

• LLM summarization — the mechanical extract (first user prompt + numerical outcomes) is good enough for v1. LLM recaps need provider config the plugin doesn't have access to; will follow up once the small-model hook is wired.
• Latency AC (< 3s for 5 sessions) — mechanical extraction is ~instant; the latency criterion is satisfied trivially without parallelism.

Test results

• 26 new tests pass (filtering, outcome extraction, composition, caching, graceful degradation)
• Full fast suite: 106/106 files, 1505/1505 tests pass
• 2 pre-existing flaky E2E failures (binary server timeout) — unrelated

Closes #523

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