Add implement-spec skill (in-progress) with its bucket docs

The skill itself takes a spec plus its tickets and drives them to one
PR, reading the tickets as a task graph so implementer subagents can run
concurrently across the ready frontier.

Documentation duties for the in-progress bucket:

- List it in skills/in-progress/README.md (flat list, name linked to its
  SKILL.md), the one entry every skill in a bucket must have. It stays
  out of the top-level README and .claude-plugin/plugin.json, and gets no
  docs page, as the bucket requires.
- Add a changeset, so the release notes carry it.
- Match the bucket's openai.yaml style in short_description: a short verb
  phrase, no closing period.

Also ignore .claude, which holds settings.local.json and agent worktrees
that should never be committed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1 +1,2 @@
 node_modules
+.claude