repo commands: --cache-dir naming a regular file exits 1 with a bare [Errno 17] File exists, not exit 2 naming the flag #765

The repo commands pass --cache-dir to open_source (dokimi/dokimi/commands/repo.py:624 _cache_dir), which calls ensure_clone (dokimi/dokimi/repos/source.py:487, cache_dir.mkdir(parents=True, exist_ok=True)) for a remote source.

Repro: python cli.py repo fetch file:///tmp/nothere@HEAD --cache-dir /tmp/cachefile, where /tmp/cachefile is a regular file. Output: ✗ [Errno 17] File exists: '/tmp/cachefile', exit 1 (with --json: "status": "failed", "reason": "exit 1: see stderr"). Calling ensure_clone(url, Path('/tmp/cachefile')) directly raises FileExistsError. There is no traceback, but a bad flag value reads as a failed run: the message does not name --cache-dir, and the exit code is 1 where bad input is 2.

The same cause as #701 (an unusable cache directory reaching an unguarded mkdir), in a different cache.

Fix options: validate --cache-dir in _cache_dir and raise InputError naming the flag and path (the check usable_cache_dir in commands/novelty_run.py can move to a shared place and be reused), or catch OSError in ensure_clone and re-raise as SourceError naming the directory. The first fails before any clone starts.

Found while fixing #701. The fix's PR is #766.