/compact — provider compaction + summary banner #123

Scope

Implement the /compact slash command (originally issue #87). Removed from PR #108 to be implemented separately.

Requirements (from #87)

1. ProviderAdapter interface gains a compactThread method (returns summary text)
2. CodexSessionRuntime implements compactThread via Codex JSON-RPC
3. OpenCodeAdapter implements compactThread via v2 SDK session.compact()
4. ClaudeAdapter and CursorAdapter keep "not supported" stubs (no programmatic compaction API)
5. thread.context.summarize command + thread.context-summarized event schemas in contracts
6. ContextTrimPoint gains optional summary field (and compactDurationMs)
7. Server-side: call compactThread → dispatch summarize → dispatch trim with summary
8. Session bootstrap after /compact: summary text sent as initial context to restarted provider session
9. /compact parses in composer slash command system
10. ContextSummaryBanner component renders above trim divider
11. If provider compaction fails, trim still proceeds (no summary)
12. Compaction duration recorded on trim point
13. Tests: decider, reactor fallback, ContextSummaryBanner rendering, session bootstrap with summary

Open Design Decisions

• Domain model unification: delete thread.context-summarized event, fold compactDurationMs into ContextTrimPoint (see review on feat: provider compaction + context trim + archive-and-new + review flow #108 for analysis)
• Decider should not fabricate empty data on thread.context-compacted event

Related

• Parent PRD: PRD: Context Trimming Commands (/clear, /compact, /new) #82
• Original spec: /compact — provider compaction + summary banner #87
• Original PR (stripped): feat: provider compaction + context trim + archive-and-new + review flow #108