Implement Branch discovery, occupancy, and safe switching

[codeacme17]

Tracks #1.

Summary

Implement cached Branch discovery, Branch Occupancy, and safe switching to existing Local or Remote-tracking Branches.

Scope

• Search separate Local and cached Remote-tracking Branch groups; exclude tags and remote symbolic HEAD aliases.
• Use opaque exact ref IDs and show Remote-qualified names.
• Compute Branch Occupancy across every registered Worktree.
• Permit switching only in a fully clean Worktree with no Conflict or In-progress Git Operation.
• Allow clean Detached HEAD to Local Branch switching; warn before leaving a commit not reachable from another named ref.
• Disable an occupied Local Branch, identify the occupying Worktree, and offer navigation to it.
• Allow the narrow Remote-tracking exception: create a same-name Local Tracking Branch only when target, name, local collision, upstream, and occupancy checks all pass.
• Revalidate Worktree revision, topology/occupancy, cleanliness, HEAD, and target ref immediately before execution.
• Run through the Repository Branch lane and clear stale file/diff selection after success.

Acceptance criteria

• Clean switches succeed; dirty, conflicted, and In-progress Worktrees are blocked without stash/discard/carry behavior.
• A Branch checked out in another Worktree cannot be overridden and the correct Worktree is identified.
• Remote-tracking selection creates only the permitted same-name Tracking Branch and never rewrites an existing Branch/upstream.
• Branch discovery does not Fetch implicitly.
• Two simultaneous switch requests cannot race Branch Occupancy and are never silently queued.
• External occupancy/ref changes produce a stale rejection and refreshed state.

Out of scope

• Arbitrary Branch creation, rename, deletion, force checkout, or automatic Fetch.

Dependencies

• Discover registered Worktrees with generation-safe identity #6, Implement versioned refresh, operation lanes, and reconciliation #7, and Build the adaptive Repository and Worktree overview #8.

PRD coverage

• FR6; AC-11–13, AC-20–22.

Verification

• Real multi-Worktree occupancy tests, clean/dirty/detached/unreachable fixtures, Remote-tracking collision matrices, and Branch-lane races.