feat(migration): add merged_into_project column for worktree adoption

Nullable pointer on observations and session_summaries that lets a
worktree's rows surface under the parent project's observation list
without data movement. Self-idempotent via PRAGMA table_info guard;
does not bump schema_versions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/services/sqlite/migrations/runner.ts

@@ -37,6 +37,7 @@ export class MigrationRunner {
     this.addSessionCustomTitleColumn();
     this.createObservationFeedbackTable();
     this.addSessionPlatformSourceColumn();
+    this.ensureMergedIntoProjectColumns();
   }
 
   /**
@@ -922,4 +923,33 @@ export class MigrationRunner {
 
     this.db.prepare('INSERT OR IGNORE INTO schema_versions (version, applied_at) VALUES (?, ?)').run(25, new Date().toISOString());
   }
+
+  /**
+   * Ensure merged_into_project columns + indices exist on observations and session_summaries.
+   *
+   * Self-idempotent via PRAGMA table_info guard — does NOT bump schema_versions.
+   * Supports merged-worktree adoption: a nullable pointer that lets a worktree's rows
+   * be surfaced under the parent project's observation list without data movement.
+   */
+  private ensureMergedIntoProjectColumns(): void {
+    const obsCols = this.db
+      .query('PRAGMA table_info(observations)')
+      .all() as TableColumnInfo[];
+    if (!obsCols.some(c => c.name === 'merged_into_project')) {
+      this.db.run('ALTER TABLE observations ADD COLUMN merged_into_project TEXT');
+      this.db.run(
+        'CREATE INDEX IF NOT EXISTS idx_observations_merged_into ON observations(merged_into_project)'
+      );
+    }
+
+    const sumCols = this.db
+      .query('PRAGMA table_info(session_summaries)')
+      .all() as TableColumnInfo[];
+    if (!sumCols.some(c => c.name === 'merged_into_project')) {
+      this.db.run('ALTER TABLE session_summaries ADD COLUMN merged_into_project TEXT');
+      this.db.run(
+        'CREATE INDEX IF NOT EXISTS idx_summaries_merged_into ON session_summaries(merged_into_project)'
+      );
+    }
+  }
 }