Handle rebase-in-progress detection via state directories and catch rebase errors

REBASE_HEAD can linger after an interrupted rebase, causing false positives.
Checking rebase-merge/ and rebase-apply/ directories matches how git itself
determines rebase state. Also surface rebase failures in the CLI spinner
instead of letting them propagate as unhandled exceptions.

src/commands/rebase.ts

@@ -21,7 +21,13 @@ export async function action(branch: string) {
   await $`git -C ${root} fetch origin ${main}`.nothrow().quiet()
   fetchSpin.text = `Rebasing onto origin/${main}`
 
-  let conflicts = await git.rebase(`origin/${main}`, worktree)
+  let conflicts: string[]
+  try {
+    conflicts = await git.rebase(`origin/${main}`, worktree)
+  } catch (err) {
+    fetchSpin.fail(String((err as Error).message ?? err))
+    process.exit(1)
+  }
   if (conflicts.length === 0) {
     fetchSpin.succeed(`Rebased ${branch} onto ${main}`)
     return

src/git.ts

@@ -153,9 +153,10 @@ export async function abortMerge(cwd: string): Promise<void> {
 
 /** Rebase the current branch onto another. Returns conflicted file paths, or empty array if clean. */
 export async function rebase(onto: string, cwd: string): Promise<string[]> {
-  // Bail early if a rebase is already in progress
-  const inProgress = await $`git -C ${cwd} rev-parse --verify --quiet REBASE_HEAD`.nothrow().quiet()
-  if (inProgress.exitCode === 0) {
+  // Bail early if a rebase is already in progress (check for rebase state directories, not REBASE_HEAD which can be stale)
+  const rebaseMerge = (await $`git -C ${cwd} rev-parse --git-path rebase-merge`.nothrow().quiet().text()).trim()
+  const rebaseApply = (await $`git -C ${cwd} rev-parse --git-path rebase-apply`.nothrow().quiet().text()).trim()
+  if (existsSync(rebaseMerge) || existsSync(rebaseApply)) {
     throw new Error(`A rebase is already in progress. Run "git -C ${cwd} rebase --abort" to cancel it first.`)
   }