Replace write_result's direct filesystem write with commit_result_via_git,
which clones the ops repo into a scratch directory, writes the result file,
commits as vault-bot, and pushes. This removes the requirement for a shared
bind-mount between the dispatcher container and the host ops-repo clone.
- Idempotent: skips if result.json already exists upstream
- Retry loop: handles push conflicts with rebase-and-push (up to 3 attempts)
- Scratch dir: cleaned up via RETURN trap regardless of outcome
- Works identically under docker and future nomad backends
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