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/services/sqlite/timeline/queries.ts

@@ -8,6 +8,7 @@
 import type { Database } from 'bun:sqlite';
 import type { ObservationRecord, SessionSummaryRecord, UserPromptRecord } from '../../../types/database.js';
 import { logger } from '../../../utils/logger.js';
+import { OBSERVER_SESSIONS_PROJECT } from '../../../shared/paths.js';
 
 /**
  * Timeline result containing observations, sessions, and prompts within a time window
@@ -210,10 +211,10 @@ export function getAllProjects(db: Database): string[] {
     SELECT DISTINCT project
     FROM sdk_sessions
     WHERE project IS NOT NULL AND project != ''
-      AND project != 'observer-sessions'
+      AND project != ?
     ORDER BY project ASC
   `);
 
-  const rows = stmt.all() as Array<{ project: string }>;
+  const rows = stmt.all(OBSERVER_SESSIONS_PROJECT) as Array<{ project: string }>;
   return rows.map(row => row.project);
 }