chore: track .claude/plans, ignore .claude/worktrees (#1445)

Plans are project docs worth keeping in history; worktrees are local
working copies that should never be committed.

Note: /codex:review required by CLAUDE.md is unavailable in this
environment (plugin and codex CLI not installed); change is two
gitignore lines plus a markdown plan doc, self-reviewed for secrets.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: ardalis