Rebase stacks onto the latest remote trunk

[skarim]

• Fixes gh stack sync merged approved PRs into bottom branch #155
• Fixes gh stack rebase doubles remote prefix for remote-qualified trunk #176
• Addresses Including the root branch when doing the rebase #215

Summary

gh stack rebase and gh stack sync could report success while leaving a stack on a stale trunk, particularly when the local trunk was checked out in another worktree, had diverged from the remote, or the fetch failed.

This change fetches the trunk explicitly and rebases onto the fresh remote ref when the local branch cannot be updated, while preserving locally-ahead and local-only trunks. It also treats rebases that git refuses to start as failures and verifies the resulting ancestry before reporting success or pushing from sync.

Testing

• go vet ./...
• go test -race -count=1 ./...
• Validated rebase and sync against a six-layer stack with main locked in another worktree
• Validated failed-fetch and deleted-trunk behavior without moving stack branches

[Copilot]

Pull request overview

Updates rebase and sync flows to target the freshly fetched remote trunk and fail safely when rebases cannot start or ancestry verification fails.

Changes:

• Resolves and fetches an authoritative trunk target.
• Adds fatal rebase-start error handling and post-rebase verification.
• Extends recovery state, mocks, and integration tests.

Show a summary per file

File	Description
cmd/rebase.go	Uses resolved trunk targets and verifies ancestry.
cmd/rebase_test.go	Updates rebase mocks.
cmd/sync.go	Rebases and verifies before pushing.
cmd/sync_test.go	Updates sync ancestry behavior.
cmd/trunk_target_test.go	Tests trunk resolution and failure paths.
cmd/utils.go	Implements trunk targeting and stack verification.
cmd/utils_test.go	Updates helper tests.
internal/git/git.go	Adds rebase-start errors and delegates.
internal/git/gitops.go	Adds exact branch fetch and upstream lookup.
internal/git/mock_ops.go	Extends Git mocks.
internal/git/rebase_start_test.go	Adds Git integration coverage.
internal/modify/apply.go	Handles refused modify rebases as fatal.

Review details

Comments suppressed due to low confidence (1)

cmd/utils.go:1178

• A start refusal on a later branch can occur after earlier branches were successfully rebased. Returning only Err leaves those earlier refs rewritten without recovery metadata or restoration, so rerunning may replay commits from the old parent history. Keep the error fatal, but restore OriginalRefs in both callers or persist a recoverable partial-cascade state.

				if git.IsRebaseStartError(rebaseErr) {
					return cascadeRebaseResult{
						Rebased: result.Rebased,
						Err:     fmt.Errorf("could not start rebase of %s onto %s: %w", br.Branch, base, rebaseErr),
					}

• Files reviewed: 12/12 changed files
• Comments generated: 4
• Review effort level: Medium