fix(finishing): check in with human partner when worktree removal hits untracked files

git worktree remove refuses when the tree holds modified or untracked
files, and the skill gave no guidance for that refusal — the natural
agent response was --force, permanently destroying files that exist
nowhere else (uncommitted plans, notes, scratch work). Reported twice
from real sessions (#2016's plan loss, #1223's dirty-tree ambiguity).

Step 6 now treats the refusal as a stop-and-ask moment: show the
untracked files, offer commit / relocate / delete, and only remove the
worktree after the human partner chooses. Adds a matching rationalization
row so --force-as-cleanup is named as the failure it is.

skills/finishing-a-development-branch/SKILL.md

@@ -174,6 +174,29 @@ git worktree remove "$WORKTREE_PATH"
 git worktree prune  # Self-healing: clean up any stale registrations
 ```
 
+**If removal is refused** (`contains modified or untracked files`): the
+worktree holds files that exist nowhere else — uncommitted plans, notes,
+or scratch work. Never `--force` on your own initiative. Show your human
+partner what is at stake and ask:
+
+```bash
+git -C "$WORKTREE_PATH" status --porcelain
+```
+
+```
+Worktree removal refused — these files were never committed:
+
+<file list>
+
+1. Commit them to <branch> before cleanup
+2. Move them into <main repo root>
+3. Delete them (unrecoverable)
+
+Which?
+```
+
+Carry out the choice, then remove the worktree.
+
 **Otherwise:** The host environment owns this workspace — leave it in
 place. If your platform provides a workspace-exit tool, use it.
 
@@ -196,6 +219,7 @@ place. If your platform provides a workspace-exit tool, use it.
 | "'Yeah, get rid of it' counts as confirmation" | Only the typed word `discard` authorizes deletion. |
 | "The PR is up, so the worktree is clutter now" | PR feedback gets fixed in that worktree. It stays until the work lands. |
 | "This other worktree looks stale — I'll clean it too" | Clean up only worktrees under `.worktrees/` or `worktrees/`. Everything else belongs to the host. |
+| "Removal refused — `--force` is just finishing the cleanup" | The refusal means files exist only in that worktree. `--force` destroys them permanently. Show your human partner and ask. |
 | "The merged-result failure is probably flaky" | A failing merged result stops everything. Branch and worktree stay put while you investigate. |
 | "The base branch is obviously main" | Confirm the fork point or ask. Merging into the wrong base is expensive to undo. |
 | "The push was rejected — force-push will fix it" | A rejected push means the remote moved. Investigate; force-push only on your human partner's explicit request. |