refactor: model worktree as workspace, not branch

[sebasv]

Motivation

Grove's data model conflated worktree with branch — Worktree.branch: String served as both the rendered label and the worktree's identity. That hid two concrete bugs:

1. Detached-HEAD linked worktrees were silently dropped from the sidebar. Mid-rebase, mid-bisect, or after an explicit commit checkout, the worktree disappeared from grove until HEAD landed on a branch again. (git::list_worktrees returned None for the entry.)
2. git switch inside a worktree's terminal broke the active selection. ActiveWorktreeId was keyed by (repo, branch), so when the branch changed underneath, grove lost the row.

Both fall out of the same conflation. The worktree's persistent identity is its path; what HEAD points at is the current state — observable, mutable, not the worktree itself.

What changed

• Worktree.head: HeadRef = Branch(name) | Detached(short_oid) replaces branch: String. Detached HEAD is now first-class — listed in the sidebar with a (detached) <7-char-oid> label.
• ActiveWorktreeId { path: PathBuf } replaces { repo, branch }. Resolution is by path match. Persisted-state schema bumped to v2 (one-time drop of v1 active selection).
• RepoDirty re-lists worktrees and remaps state by path before refreshing statuses. git switch, git worktree add, git worktree remove performed in a terminal now flow into the sidebar automatically — no manual refresh.
• PR polling and branch deletion skip detached-HEAD worktrees — they have no branch to operate on.

Tests

• list_worktrees_includes_detached_linked_worktree — detached HEAD lands as HeadRef::Detached rather than disappearing.
• reconcile_worktrees_picks_up_branch_switch_in_terminal — running git switch in the worktree updates the label and keeps the active selection (path-keyed).
• active_worktree_survives_branch_switch_inside_worktree — pure unit test of the same invariant.

Deferred

Called out as follow-ups, not in this PR:

• Path-basename suffix when label diverges from path (helps the rename case where wt-feat-foo/ is now on bugfix/y).
• A dedicated HEAD watcher — the existing recursive .git/ watcher already covers .git/worktrees/<name>/HEAD, so it's not needed yet.

Test plan

• Open grove on a repo with a normal worktree; confirm sidebar still renders the same label.
• Inside a worktree's terminal, run git switch -c some-other-branch; confirm the sidebar label updates within ~150ms (FS-watcher debounce) and the active selection sticks.
• In a worktree, run git checkout HEAD~1 (detaches HEAD); confirm the row stays put and now shows (detached) <oid>.
• Create a worktree via grove; confirm it still appears in the sidebar (no regression to try_create_worktree_modal).
• Delete a worktree on a detached HEAD via grove; confirm the worktree is removed but no branch-deletion is attempted.
• Delete ~/Library/Application Support/grove/state.toml is NOT required; the schema bump drops v1 state on its own with a warning.

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