Auto-push, sync messaging, and stale-ref sync fixes

[brdv]

Summary

Dogfooding friction fixes and two sync correctness bugs.

Auto-push unpushed commits in new/submit, clarify sync step messaging, fix stale local main ref in sync, and fix chained sync regression for multi-branch stacks.

Auto-push: When a branch already has an upstream but has unpushed local commits, new and submit now auto-push before creating the PR. This removes the manual git push step that was required between stacked branch operations — without it, GitHub would reject PR creation with "No commits between base and head."

Sync messaging: When old and new base are the same branch name (after a parent PR merge), the message now reads "dropping already-upstream commits" instead of the confusing "onto main (from main)."

Stale local main (Fix 3): stck sync fetches from origin but resolve_base_ref() preferred local refs over remote refs. After a PR merges on GitHub, local main is stale, so the rebase --onto target and merge-base resolved to stale values — making sync a no-op. Fixed by flipping resolve_base_ref() to prefer refs/remotes/origin/<branch>.

Chained sync (Fix 4): The stale-main fix overcorrects for chained syncs. When syncing a 3+ branch stack, step N rebases a branch locally but the remote ref is stale (not yet pushed). Step N+1's --onto target resolves to that stale remote ref — a no-op that leaves the descendant diverged. Fixed by tracking rebased branches in a HashSet during the sync loop and using local refs for subsequent steps.

Changelist

• src/gitops.rs — Add push_branch() for safe fast-forward pushes; add resolve_onto_ref() wrapper; flip resolve_base_ref() to prefer remote refs
• src/commands.rs — run_new() and run_submit() auto-push when upstream exists but branch needs push
• src/commands.rs — Sync step message branches on old_base_ref == new_base_ref to show "dropping already-upstream commits"
• src/commands.rs — Sync loop uses resolve_onto_ref() for --onto target; tracks rebased_in_this_sync to use local ref for chained steps
• tests/harness/mod.rs — Add stub handlers for plain git push origin <branch>, STCK_TEST_LOCAL_MAIN_SHA/STCK_TEST_REMOTE_MAIN_SHA env vars, split merge-base handling for local vs remote main
• tests/new.rs — Add tests for auto-push and skip-push-when-up-to-date
• tests/submit.rs — Add test for auto-push
• tests/sync.rs — Add sync_uses_remote_main_when_local_main_is_stale and sync_chained_rebase_uses_local_ref_for_previously_rebased_branch; update existing chained sync assertions

Checklist

• Changes are minimal and focused for this milestone
• No breaking CLI changes unless explicitly intended
• Errors are user-facing, actionable, and prefixed with error:
• Added/updated tests for behavior changes
• Updated docs/plan if behavior or scope changed

QA

• stck new a → commit → stck new b → should auto-push a and create PR without error
• stck submit on a branch with unpushed commits → should auto-push then create PR
• Merge bottom PR in a stack → stck sync → step message says "dropping already-upstream commits"
• Merge bottom PR in a 2-branch stack → stck sync → child rebases onto updated main (not a no-op)
• Merge bottom PR in a 3-branch stack → stck sync → both descendants rebase correctly; no conflicts on GitHub