auth.json corruption causes startup crash (sub-cause of #27530) #29628

Summary

opencode crashes on startup when auth.json is in a corrupted state (whitespace-only, truncated, or otherwise unparseable JSON). The crash is fatal -- the user cannot launch opencode at all until they manually delete or restore the file.

This is a sub-cause of umbrella issue #27530, which tracks multiple distinct startup-failure modes (provider-API 5xx, #-in-instructions parser crash, SSL/config failures). This issue narrows the scope to only the corrupted-auth.json case so it can be addressed independently.

Reproduction

1. Open ~/.local/share/opencode/auth.json (Linux/Mac) or %LOCALAPPDATA%\opencode\auth.json (Windows)
2. Replace the contents with whitespace, an empty string, or invalid JSON like { (simulating an interrupted write or disk-full event)
3. Launch opencode
4. Expected: opencode treats the corrupted file as if no auth exists, prompts the user to log in fresh, continues startup.
5. Actual: opencode throws an unhandled JSON parse exception during auth initialization, exits with a stack trace.

Why this happens

Interrupted writes to auth.json (Ctrl+C during save, sudden shutdown, disk-full, antivirus quarantine) leave the file in a partially-written state. The current auth loader calls JSON.parse() on the raw contents without a try/catch + recovery path, so any malformed content crashes the whole process.

Proposed fix

Treat corrupted auth.json as logically equivalent to a missing/empty auth.json -- i.e. proceed with no cached credentials and let the user re-authenticate. PR #29562 implements this: wraps the load in a try/catch that, on parse failure, logs a warning and returns an empty auth object.

Linked PR

PR #29562 (this issue's fix): #29562

Related

Part of umbrella issue #27530 (which also tracks 3 other unrelated startup-failure modes still open for separate fixes).