v2.0.4-hardening: fuzz suite + integration tests + production readiness

Phase 8 of v2.0. Final hardening pass before production tag.

Path-guard fuzz suite (34 tests): traversal attacks (../ all depths,
encoded %2e%2e, null bytes, absolute escapes, prefix-without-separator,
backslash), secret-file deny list (.env, *.pem, id_rsa*, *.key,
credentials.json, *.kdbx, .netrc), valid-path positives, edge cases
(empty, whitespace, very long, triple-dot, multiple slashes).

write_guard.ts hardened: added null-byte rejection and whitespace-only
rejection (previously only checked empty string).

Pending-changes integration test skeleton: 4 tests covering the full
queue→apply→rewind cycle against a real DB + filesystem. Gated on
DATABASE_URL via describe.runIf (same pattern as apps/server's
tool_cost_stats.test.ts). Skips cleanly when unset.

57 tests passing (23 existing + 34 fuzz), 4 integration skipped.
All builds clean. All services healthy.

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

apps/coder/src/services/write_guard.ts

@@ -54,10 +54,14 @@ export function isSecretPath(filePath: string): boolean {
  * checks the result stays within projectRoot.
  */
 export function resolveWritePath(projectRoot: string, filePath: string): string {
-  if (!filePath || filePath.length === 0) {
+  if (!filePath || filePath.trim().length === 0) {
     throw new WriteGuardError('file path is required');
   }
 
+  if (filePath.includes('\x00')) {
+    throw new WriteGuardError('file path contains null byte');
+  }
+
   const candidate = filePath.startsWith('/') ? filePath : resolve(projectRoot, filePath);
   const normalized = resolve(candidate); // normalizes ../ segments