fix: prune stale tracking refs before pushing so re-split branches are not rejected as stale

[vitali87]

Summary

Split branch names are reused across runs (pr-split/<ns>/<group>). After pr-split merge (gh deletes the head branch), clean, or GitHub auto-deleting head branches, the remote branch is gone but refs/remotes/origin/pr-split/… still points at the old head — so every git push --force-with-lease of the next split was rejected with "stale info", PRCreationError fired, and the plan was saved with branches but no PRs. Re-splitting the same dev branch was impossible without a manual git fetch --prune.

• _push_and_create_prs now runs git remote prune origin once before the push pool (warning-only if origin is unreachable). It drops only refs whose remote branch is gone — deliberately not fetch --prune, which would refresh live refs and silently defeat the lease against a reviewer's fixup push to a reused branch (the review caught that in the first version).
• delete_branch(remote=True) drops refs/remotes/origin/<branch> after the remote delete, covering the "already gone on origin" case.

Stacked on #156 (stack #152 = #55→#151→#156→this).

Test plan

• real-git TestStaleRemoteTrackingRefs: remote branch deleted behind our back → push rejected as stale → prune → push succeeds; a third-party fixup push is still rejected after prune (lease intact); remote delete drops the tracking ref; already-deleted remote still drops it
• TestPushPrunesFirst (prune before any push); mocked delete_branch tests stub the helper
• Review re-ran its probe (third-party push, deleted branch, single-branch clone, unreachable origin)
• 464 tests pass, ruff clean
• Local review: 5/5 (after one fix→re-review round)

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