feat(engineering): add implement-isolation skill, refine implement-issue

- Add /skill:implement (implement-isolation) — ticket claim, worktree
  isolation, tdd, verify, code-review, ship loop
- Fix implement-issue: worktree path to issue-<N>-<repo-slug>,
  label semantics (remove-others), skill ref formatting, drop stale
  closing note

common/engineering/implement/SKILL.md

@@ -0,0 +1,47 @@
+---
+name: implement-isolation
+description: "Implement a piece of work based on a spec or set of tickets in isolation."
+disable-model-invocation: true
+---
+
+## Invocation
+
+/skill:implement <N> [--base <branch>] [--force]
+
+- `<N>` — required, the ticket/issue number
+- `--base <branch>` — optional, target base branch (default: repo default branch)
+- `--force` — optional, allow overwriting an existing `worktree`
+
+Triage labels follow the vocabulary in `docs/agents/triage-labels.md`.
+
+## Process
+
+### 1. Claim the ticket
+
+Label the issue `in-progress`, remove all other labels.
+
+### 2. Isolate the workspace
+
+```bash
+git worktree add -b <branch-name> ../issue-<N>-<repo-slug> <base>
+```
+
+Change into the new worktree directory.
+
+### 3. Implement
+
+Drive every acceptance criterion through **red-green-refactor**: write the test first (red), make it pass (green), clean up (refactor). No code without a failing test first.
+
+### 4. Verify
+
+Run the full test suite. Every test must pass.
+
+### 5. Review
+
+Run /code-review against the base branch. Fix every finding before proceeding.
+
+### 6. Ship
+
+Push the branch, open a PR. The PR description must include a link to the ticket and a key-changes list.
+
+Comment on the issue: `PR opened: <url>`. Label the issue `needs-review`, remove all other labels.