chore(repo): ignore .worktrees/ so an agent worktree inside the checkout is not untracked (#17468)

`.gitignore:129` carried `.claude/worktrees/` and nothing else, so a linked
worktree created at `<checkout>/.worktrees/NAME` — one of five registered
worktrees in a live container chose that path — showed up as `?? .worktrees/`.
A stop hook that asks for untracked files to be committed then points at the
one kind of path that must never be committed: the directory holds a `.git`
FILE whose content is `gitdir: /abs/path/.git/worktrees/NAME`, an absolute
pointer into one container's administrative state, plus a whole second
checkout of the repository.

Measured in a dedicated worktree, before and after:

  git worktree add .worktrees/scratch HEAD
  git status --porcelain          before: `?? .worktrees/`   after: clean
  git check-ignore -v .worktrees  before: exit 1 (no match)  after: exit 0,
                                  `.gitignore:131:.worktrees/`

The entry is the exact directory name, dot-prefixed and directory-only, in the
same unanchored form as its neighbour `.codex/`. It matches `.worktrees/` at
any depth and nothing else: `worktrees/`, `my-worktrees/`, `.worktrees-backup/`
and `docs/worktrees/` all still read as not ignored.

Claude-Session: https://claude.ai/code/session_01YKEjmbYNvYWJvWGSWx26zK

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

Author: os-litant

.gitignore

@@ -126,4 +126,6 @@ examples/app-crm/storage/
 
 # Local agent tooling
 .codex/
-.claude/worktrees/
+.claude/worktrees/
+# an agent worktree is git plumbing, never repo content: a `.git` FILE reading `gitdir:` plus a whole second checkout
+.worktrees/