perf(bootstrap): lazy thread hydration with listing snapshot

[tyulyukov]

Summary

Implements a two-phase bootstrap strategy to unblock sidebar rendering while background-loading thread data:

• Phase 1 (Listing Snapshot): Fast initial load fetches projects + lightweight OrchestrationThreadSummary (metadata, session, latest turn, pre-computed latestUserMessageAt). Skips messages, activities, checkpoints, and proposed plans. Sets bootstrapComplete = true to render sidebar immediately.
• Phase 2 (Lazy Hydration): When user navigates to a thread, full data is fetched on-demand via getThread() and merged into the store. The hollow thread structure persists until hydrated.

Additional improvements:

• Added database indexes on projection_thread_messages and projection_turns for listing snapshot query performance
• Sidebar shows skeleton loaders during bootstrap instead of blank state
• Text reveal animation now triggers scroll-to-reveal for large messages during hydration
• Sidebar summary fields (hasPendingApprovals, hasPendingUserInput, hasActionableProposedPlan) default to false and are corrected by real-time domain events within seconds

Architecture:

• New server-side getListingSnapshot() and getThread(threadId) methods in ProjectionSnapshotQuery
• WebSocket endpoints: orchestration.getListingSnapshot and orchestration.getThread
• Frontend isThreadHydrated() check in _chat.$threadId.tsx triggers hydration on navigation
• Structural sharing preserved in store so unhydrated threads don't trigger unnecessary re-renders

Testing

• bun fmt passes
• bun lint passes
• bun typecheck passes
• Manual: Sidebar renders immediately with skeleton loaders on cold start
• Manual: Clicking a thread fetches and merges full data without UI jank
• Manual: Real-time domain events update sidebar summary fields after hydration
• Manual: Text reveal animation scrolls large messages into viewport during hydration
• Manual: Switching between threads preserves hydrated data; hollow threads are skipped
• Manual: Reconnect/snapshot recovery still uses full getSnapshot() as fallback