Stop reporting rebases that git refused to run as successes

[skarim]

The bug

gh stack rebase and gh stack sync could print a complete success report while leaving the stack exactly where it was — matching the user reports:

✓ Trunk main fast-forwarded to 84bacb3
Stack detected: (main) <- phil/trd-mark-edge-drills <- phil/implement-in-place-apis
✓ Rebased phil/trd-mark-edge-drills onto main
✓ Rebased phil/implement-in-place-apis onto phil/trd-mark-edge-drills
All branches in stack rebased locally with main

…while git merge-base --is-ancestor main HEAD still said main was not an ancestor.

Root cause

tryAutoResolveRebase in internal/git/git.go — the shared error path for every rebase — returned nil whenever no rebase was in progress:

for i := 0; i < 1000; i++ {
    if !IsRebaseInProgress() {
        return nil          // <- "success"
    }

That early return is only a valid success signal after an auto---continue. On the first check it means git rebase exited non-zero without ever starting. Verified against real repos, all of these hit it:

Cause	git says
Dirty working tree	error: cannot rebase: You have unstaged changes.
Branch checked out in another worktree	fatal: 'b2' is already used by worktree at …
Trunk missing locally	fatal: invalid upstream 'main'
Stale rebase in progress	fatal: … rebase-merge directory already exists

Every one returned nil, so cascadeRebase printed ✓ Rebased X onto Y and carried on. It affected rebase, sync, and modify.

Reverting just this one change reproduces the reported log byte-for-byte in the new integration test.

Fixes

Silent failures

• New typed *git.RebaseStartError for a rebase that never started, plus an up-front check so an already-running rebase isn't mistaken for this rebase's conflicts.
• cascadeRebase (and modify) treat it as fatal rather than a conflict — no bogus rebase-state file, and git's own message is surfaced.
• rebase/sync gain the preflight checks modify already had (no rebase in progress, clean working tree), with --autostash to opt out by passing --autostash straight through to git.
• After every cascade, verifyStacked() confirms each branch really sits on top of its parent and fails instead of printing the success summary. sync checks before pushing, so an unrebased stack is never force-pushed.

Duplicate commits

The cascade passed the parent's current tip as the --onto upstream with no staleness guard, so a parent amended, reordered, or squash-merged out of band had its old commits replayed onto the child. The merged-PR path guarded this but fell back to merge-base(newBase, branch), which replays squashed commits. Both paths now use resolveOntoOldBase() — the latest commit the child genuinely contains. Without it the new test hits a spurious conflict on the parent's own file; with it the rebase is clean.

Found alongside

• git rebase <option> --continue is a usage error (exit 129), so --preserve-dates made rebase --continue fail every time. git persists the option in .git/rebase-merge, so --continue alone honors it.
• sync never called ensureLocalTrunk before rebasing onto the trunk, and printed "Fetched latest changes" even when the fetch failed.
• A trunk whose remote branch is gone now says so instead of silently rebasing onto a stale local trunk.

Testing

• Real-git integration tests (cmd/rebase_integration_test.go, internal/git/gitops_test.go) running the actual commands against real repos with real remotes: trunk is pulled in, dirty tree rejected, --autostash rebases and restores, foreign worktree fails loudly, amended parent doesn't duplicate commits.
• Table-driven MockOps tests for failure classification, both preflights, --autostash pass-through, and post-cascade verification.
• Unit tests for resolveOntoOldBase and verifyStacked.
• go vet ./... and go test -race -count=1 ./... pass.

Not included

#225's trunk migration (re-pointing a stack whose trunk branch was squash-merged and deleted) is deferred to a follow-up, per discussion — it needs more nuanced semantics around changing a stack's trunk. This PR only makes that situation visible rather than a silent no-op.

[Copilot]

Pull request overview

Prevents failed Git rebases from being reported as successful and strengthens stack verification.

Changes:

• Adds typed rebase-start failures and post-rebase ancestry checks.
• Adds --autostash and safer sync/rebase preflights.
• Improves old-base selection, fetch diagnostics, documentation, and tests.

Show a summary per file

File	Description
internal/modify/apply.go	Handles rebase start failures during modify.
internal/git/gitops.go	Adds rebase flags and fetch error handling.
internal/git/git.go	Classifies rebase failures and fixes continuation.
internal/git/gitops_test.go	Adds real-Git rebase tests.
cmd/utils.go	Adds preflights, verification, and old-base resolution.
cmd/utils_test.go	Tests ancestry verification and base selection.
cmd/rebase.go	Adds autostash and post-cascade verification.
cmd/rebase_test.go	Tests rebase failure and preflight paths.
cmd/rebase_integration_test.go	Adds command-level Git integration coverage.
cmd/sync.go	Prevents pushing after failed verification.
cmd/sync_test.go	Tests sync failure and autostash behavior.
docs/src/content/docs/reference/cli.md	Documents updated CLI behavior.
docs/src/content/docs/guides/workflows.md	Documents autostash workflows.

Review details

• Files reviewed: 13/13 changed files
• Comments generated: 4
• Review effort level: Medium