Fix branch selector confusion during merge/rebase conflicts

[posthog]

Summary

When the local repo is mid-rebase (or merge/cherry-pick/revert), the new task input's branch selector behaved badly in three ways. This PR cleans all three up.

#	Symptom	Cause	Fix
1	First dropdown item was literally (no	simple-git's branchLocal() parser greedily mis-parses git's * (no branch, rebasing main) pseudo-branch line and returns (no as a "branch name"	Switched getAllBranches to git for-each-ref refs/heads/ so only real branches are returned
2	Label said "No branch" with no explanation	getCurrentBranch returns null during rebase (HEAD is detached on the rebase pseudo-ref)	New getGitBusyState tRPC query reports rebase/merge/cherry-pick/revert; the selector now shows "Rebasing", "Merging", etc. when there's no current branch
3	Picking a branch threw Failed to checkout …	git refuses checkout while an operation is in progress	The selector is disabled in local mode when busy, with a tooltip: "Rebasing in progress — finish or abort it to switch branches."

The busy-state probe (rebase-merge/, MERGE_HEAD, CHERRY_PICK_HEAD, REVERT_HEAD) was already implemented inside CaptureCheckpointSaga. It's now extracted to inspectGitBusyState in queries.ts so the saga and the new public getGitBusyState(baseDir) share one implementation — no behavior change for the saga; existing 18 checkpoint tests still pass.

Worktree and cloud modes are unaffected (busyState only applies when !isSelectionOnly).

Test plan

• New unit test covers getAllBranches filtering the (no pseudo-branch during a rebase conflict
• New unit test covers getGitBusyState reporting { busy: true, operation: "rebase" } mid-rebase
• All 18 existing checkpoint sagas tests still pass after extracting inspectGitBusyState
• Manually verify in dev: open new task input while local repo is mid-rebase → selector shows "Rebasing" and is disabled with tooltip; dropdown contains only real branches

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