chore(hygiene): NUL byte in repository.ts, hourly mirror warning, dead deploy workflows removed #5511

Findings 3 + 4 from the Vapron instance's briefing, fixed

1. src/git/repository.ts is a text file again. One raw NUL byte inside a cache-key template literal made grep/file(1) treat ~3,000 lines of load-bearing git plumbing as binary (it bit me mid-investigation this week too — and it just bit a third time by making the first attempt at THIS pull request fail: Postgres refuses NUL in text, so the PR body couldn't even mention the byte literally). Replaced with the backslash-u0000 escape — byte-identical string value at runtime.
2. Zero-mirrors warning: hourly instead of every 5-minute tick. The every-tick honesty (288/day) had crossed into noise operators learn to filter — the exact failure mode the warning exists to prevent. Message unchanged, rate-limited, with an explicit "(Repeats hourly while true.)" suffix. Mirror-sync failure test still passes.
3. Deleted .github/workflows/hetzner-deploy.yml and fly-deploy.yml per Craig — Hetzner is gone for months and not coming back. heartbeat.yml (the one live GitHub job) untouched. Stale references to hetzner-deploy in rollback-deploy.ts/admin-deploys-page.tsx were already dead paths — follow-up, not smuggled into a hygiene pass.

Typecheck clean; targeted tests green.

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