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/core/src/db.ts

@@ -317,7 +317,7 @@ export class Database {
   /** Tracks transaction nesting depth for savepoint-based nested transactions. */
   private transactionDepth = 0;
 
-  constructor(private kbDir: string) {
+  constructor(kbDir: string) {
     this.dbPath = join(kbDir, "fusion.db");
 
     // Ensure .fusion directory exists