fix(cli): report persisted config read failures cleanly

[rudycelekli]

What changed

A real unreadable ifixai.yaml, or a directory at that path, currently makes ifixai run exit with a raw traceback. load_config documents a ValueError failure contract and the CLI already provides a Config error: diagnostic for that contract. Translate only filesystem OSError from reading persisted config into that existing error. YAML and schema validation remain unchanged.

Native reproduction and tests

• Actual subprocess CLI against a valid YAML file with POSIX mode000 and a directory named ifixai.yaml: 2 regression failures before; no mocked filesystem.
• After: 24 focused tests pass, including 3 new controls (readable config, missing config, malformed YAML) and existing saved-choice tests. Readable/missing config completes a keyless offline mock dry run. Permission test explicitly skips on Windows or root where this native permission contract is not enforced.
• Ruff0.16.9, Bandit, layout validation and all11 example fixtures pass at signed head c8f5ea8. Mypy advisory retains1 existing provider error.
• Official fork CI Python3.10/3.11/3.12 all succeed: https://github.com/rudycelekli/iFixAi/actions/runs/37540649307 . This is fork validation; upstream PR checks are separately observed after creation.

No provider, fixture or inspection behavior changes. The commit includes a verified SSH signature and DCO sign-off. AI assistance: Codex helped investigate, implement and test; native caller evidence and the complete diff were independently source-reviewed before publication.