A test pins forking when the copy fails: `fork_failed`, with no repository row or directory left behind #7175

forkRepo in src/core/repos.ts answers fork_failed when forkBareRepo fails, and no test names it.

Done:

• [ ] A new test makes the fork's target unwritable for the test, then forks through POST /api/v1/repos/:owner/:name/forks. The answer is a refusal in words, not a 500.
• [ ] No repository row exists for the fork, and no directory is left under the repos root.
• [ ] A second try, once the target is writable, succeeds with the same name.

Files: test/fork-failed.test.ts

If the test finds the behaviour wrong, fix the module it names in the same pull and say so. If the check fails in a test this pull does not touch, name it in the pull rather than widening the change.

Before pushing: bun x tsc --noEmit and the new test.