Add git worktree awareness to status line

Sessions in a linked worktree previously showed only the worktree's
directory name, so the underlying repository was unidentifiable and the
session looked like an unrelated project.

The project segment now renders as "repo | worktree". Repo identity comes
from git rev-parse --git-common-dir, so it is correct regardless of how the
worktree directory was named. A leading repo prefix is stripped from the
worktree name, and the name collapses to a bare marker when it merely
restates the branch already on screen -- matched across / _ . - separators
and type prefixes like feature/, and only when the branch is displayed.

Gated behind the new items.worktree config flag (default on). Costs one
extra timeout-bounded git call, skipped entirely outside git repos.

src/statusline-claude/statusline-config.json

@@ -6,6 +6,7 @@
     "model": true,
     "effort": true,
     "project": true,
+    "worktree": true,
     "branch": true,
     "contextBar": true,
     "contextTokens": true,