fix(engine): recoverable worktree failures + prevent nested/gitlink worktrees

Fixes two classes of task failures found while investigating stuck in-review
tasks FN-2165 (worktree base ref missing) and FN-2152 (stray .tmp-fn-2152
gitlink accidentally committed via merger amend).

FN-2165 — stale baseBranch:
- resolveWorktreeStartPoint now returns null instead of throwing
  NonRetryableWorktreeError when the stored baseBranch is gone. Caller clears
  task.baseBranch and falls back to branching from the default base (HEAD) so
  the task self-heals instead of failing permanently.
- New TaskStore.clearStaleBaseBranchReferences() nulls baseBranch on any
  dependent task when its upstream branch is deleted. Wired into
  cleanupBranchForTask (archive/delete), merger branch cleanup, self-healing
  orphan-branch sweep, executor dep-abort and conflict-cleanup paths, and
  stale-branch recovery.

Nested worktrees:
- assertWorktreePathNotNested guard in tryCreateWorktree refuses to create a
  worktree inside another registered worktree (previously produced pathological
  paths like .worktrees/green-finch/.worktrees/amber-panda when rootDir pointed
  at a worktree instead of the main repo).

Context-overflow recovery (FN-2182 class):
- Reduced-prompt retry budget raised from 1 → 3 within the same session.
- Adds a fresh-session requeue path when same-session retries still overflow:
  task moves back to todo with worktree retained, bounded by
  computeRecoveryDecision / MAX_RECOVERY_RETRIES. Prevents late-step context
  exhaustion from becoming terminal.

Gitlink prevention (FN-2152 class):
- .gitignore now excludes .tmp-fn-* and .tmp-kb-* so stray worktrees at the
  repo root cannot be captured by git add -A.
- Merger amend flow now scans staged entries for 160000 gitlinks and unstages
  them with a loud warning; the project uses no submodules, so any such entry
  is a bug (this is how f8f90f26 landed in HEAD as .tmp-fn-2152).

Tests: new coverage for baseBranch fallback, nested-worktree guard, and
clearStaleBaseBranchReferences. Full engine + core + dashboard + cli suites
pass (15349 tests).

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

packages/engine/src/self-healing.ts

@@ -1509,6 +1509,7 @@ export class SelfHealingManager {
       if (orphaned.length === 0) return 0;
 
       let cleaned = 0;
+      const deletedBranches: string[] = [];
       for (const branch of orphaned) {
         try {
           // Try safe delete first (-d requires branch to be merged)
@@ -1518,6 +1519,7 @@ export class SelfHealingManager {
           });
           log.log(`Deleted branch: ${branch}`);
           cleaned++;
+          deletedBranches.push(branch);
         } catch (err: unknown) {
           const errorMessage = err instanceof Error ? err.message : String(err);
           log.warn(
@@ -1531,6 +1533,7 @@ export class SelfHealingManager {
             });
             log.log(`Force-deleted branch: ${branch}`);
             cleaned++;
+            deletedBranches.push(branch);
           } catch (forceErr: unknown) {
             const forceErrorMessage = forceErr instanceof Error ? forceErr.message : String(forceErr);
             log.warn(`Failed to force-delete orphaned branch ${branch}: ${forceErrorMessage} — non-fatal`);
@@ -1539,6 +1542,16 @@ export class SelfHealingManager {
         }
       }
 
+      if (deletedBranches.length > 0) {
+        // FN-2165 regression guard: if any dependent task stored one of these
+        // now-gone branches as its baseBranch, null it so the task doesn't
+        // hard-fail at worktree creation time.
+        const cleared = this.store.clearStaleBaseBranchReferences(deletedBranches);
+        if (cleared.length > 0) {
+          log.log(`Cleared stale baseBranch on ${cleared.length} task(s): ${cleared.join(", ")}`);
+        }
+      }
+
       if (cleaned > 0) {
         log.log(`Cleaned ${cleaned} orphaned branch(es)`);
       }