feat(engine): harden review pipeline with strict scope, build retry, and E2E tests

Improve the plan→review→approve→merge agent pipeline:

- Harden verdict extraction with JSON block parsing and anchored regexes
- Consolidate legacy/new merger conflict APIs into thin deprecated wrappers
- Add configurable strict scope enforcement (strictScopeEnforcement setting)
- Add build retry with timeout to merger (buildRetryCount, buildTimeoutMs)
- Add handleChangesRequested to PrCommentHandler for review feedback loop
- Remove dead code: handleFsChange, processTaskChange, unused imports/fields
- Add E2E multi-verdict sequence tests for the full review pipeline
- Fix unused parameter warnings across engine and core packages

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/engine/src/stuck-task-detector.ts

@@ -136,7 +136,7 @@ export class StuckTaskDetector {
    * - Moves the task back to "todo" (preserving step progress)
    * - Invokes the onStuck callback
    */
-  async killAndRetry(taskId: string, timeoutMs: number): Promise<void> {
+  async killAndRetry(taskId: string, _timeoutMs: number): Promise<void> {
     const entry = this.tracked.get(taskId);
     if (!entry) return;