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/worker/PaginationHelper.ts

@@ -9,6 +9,7 @@
 
 import { DatabaseManager } from './DatabaseManager.js';
 import { logger } from '../../utils/logger.js';
+import { OBSERVER_SESSIONS_PROJECT } from '../../shared/paths.js';
 import type { PaginatedResult, Observation, Summary, UserPrompt } from '../worker-types.js';
 
 export class PaginationHelper {
@@ -107,7 +108,8 @@ export class PaginationHelper {
       params.push(project, project);
     } else {
       // Hide internal observer-session rows from the unfiltered UI list.
-      conditions.push("o.project != 'observer-sessions'");
+      conditions.push('o.project != ?');
+      params.push(OBSERVER_SESSIONS_PROJECT);
     }
     if (platformSource) {
       conditions.push(`COALESCE(s.platform_source, 'claude') = ?`);