Make kickoff recovery idempotent before Git mutations

A branch/PR kickoff helper should be idempotent before performing commits: when an issue branch and pull request already exist, detect and reuse them before creating or pushing another kickoff commit. This matters when a repository guard instructs an agent to rerun kickoff to authorize a worktree operation; otherwise the recovery path can mutate the PR and still fail. The observation applies to helper-driven Git workflows with existing branches/PRs, not first-time kickoff.

Evidence/example — 2026-09-10

In the Shield repository, the Linear progress guard rejected both a normal and detached git worktree add for an already-linked issue and told the agent to rerun kickoff. The kickoff helper pushed an empty second kickoff commit, then failed because the existing PR was discovered only at PR-creation time. Retrying worktree creation remained blocked. Provenance: local hook diagnostics, git history, and GitHub PR state observed during COR-4175/PR #840. No application code changed.

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