feat(witness): delay polecat cleanup until MR merges (gt-12hwb)

Phase 4 of local-only polecat branches: Handle conflict resolution edge case.

Problem: If polecat worktree is nuked before MR merges, the local branch
is gone and conflict resolution can't access it.

Solution: Witness now defers cleanup for polecats with pending MRs:
- HandlePolecatDone creates a cleanup wisp with "merge-requested" state
- Polecat worktree preserved until MERGED signal arrives
- HandleMerged then nukes the polecat (existing behavior)

Also updated mol-polecat-conflict-resolve.formula.toml:
- Removed fetch from origin (branches are local-only now)
- Added instructions to fetch from source polecat's worktree
- Added rig and source_polecat variables

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/formula/formulas/mol-polecat-conflict-resolve.formula.toml

@@ -145,20 +145,39 @@ git stash list              # Should be empty
 
 If dirty, clean up first (stash or discard).
 
-**2. Fetch latest state:**
+**2. Fetch latest main:**
 ```bash
 git fetch origin
-git fetch origin {{branch}}:refs/remotes/origin/{{branch}}
 ```
 
-**3. Checkout the branch:**
+**3. Locate the source polecat's worktree:**
+
+The branch is local-only (not pushed to origin). Find the source polecat path
+from the task metadata. The path follows the pattern:
+```
+~/gt/<rig>/polecats/<polecat-name>
+```
+
+Extract the source polecat name from the MR metadata:
 ```bash
-git checkout -b temp-resolve origin/{{branch}}
+bd show {{original_mr}} --json | jq -r '.description' | grep -oP 'Source polecat: \K\S+'
+```
+
+**4. Fetch the branch from the source polecat's worktree:**
+```bash
+# The source polecat's worktree still exists (cleanup is deferred until MR merges)
+SOURCE_POLECAT_PATH="$HOME/gt/{{rig}}/polecats/{{source_polecat}}"
+git fetch "$SOURCE_POLECAT_PATH" {{branch}}:{{branch}}
+```
+
+**5. Checkout the branch:**
+```bash
+git checkout -b temp-resolve {{branch}}
 ```
 
 Using `temp-resolve` as the local branch name keeps things clear.
 
-**4. Verify the branch state:**
+**6. Verify the branch state:**
 ```bash
 git log --oneline -5        # Recent commits
 git log origin/main..HEAD   # Commits not on main
@@ -383,3 +402,11 @@ required = true
 [vars.branch]
 description = "The branch to rebase (extracted from task metadata)"
 required = true
+
+[vars.rig]
+description = "The rig where the source polecat resides"
+required = true
+
+[vars.source_polecat]
+description = "The name of the polecat whose local branch contains the work (extracted from MR metadata)"
+required = true