fix(worktree): address PR review — schema guard, startup adoption, query parity

Addresses six CodeRabbit/Greptile findings on PR #2052:

- Schema guard in adoptMergedWorktrees probes for merged_into_project
  columns before preparing statements; returns early when absent so first
  boot after upgrade (pre-migration) doesn't silently fail.

- Startup adoption now iterates distinct cwds from pending_messages and
  dedupes via resolveMainRepoPath — the worker daemon runs with
  cwd=plugin scripts dir, so process.cwd() fallback was a no-op.

- ObservationCompiler single-project queries (queryObservations /
  querySummaries) OR merged_into_project into WHERE so injected context
  surfaces adopted worktree rows, matching the Multi variants.

- SessionStore constructor now calls ensureMergedIntoProjectColumns so
  bundled artifacts (context-generator.cjs) that embed SessionStore get
  the merged_into_project column on DBs that only went through the
  bundled migration chain.

- OBSERVER_SESSIONS_PROJECT constant is now derived from
  basename(OBSERVER_SESSIONS_DIR) and used across PaginationHelper,
  SessionStore, and timeline queries instead of hardcoded strings.

- Corrected misleading Chroma retry docstring in WorktreeAdoption to
  match actual behavior (no auto-retry once SQL commits).

src/shared/paths.ts

@@ -77,7 +77,7 @@ export const OBSERVER_SESSIONS_DIR = join(DATA_DIR, 'observer-sessions');
 
 // Project name assigned to observer sessions (basename of OBSERVER_SESSIONS_DIR).
 // UI queries filter this out so internal worker sessions don't pollute project lists.
-export const OBSERVER_SESSIONS_PROJECT = 'observer-sessions';
+export const OBSERVER_SESSIONS_PROJECT = basename(OBSERVER_SESSIONS_DIR);
 
 // Claude integration paths
 export const CLAUDE_SETTINGS_PATH = join(CLAUDE_CONFIG_DIR, 'settings.json');