Fixes two corruption patterns that break API keys during updates:
1. Concatenated KEY=VALUE pairs on a single line due to missing newlines
(e.g. ANTHROPIC_API_KEY=sk-...OPENAI_BASE_URL=https://...). Uses a
known-keys set to safely detect and split concatenated entries without
false-splitting values that contain uppercase text.
2. Stale KEY=*** placeholder entries left by incomplete setup runs that
never get updated and shadow real credentials.
Changes:
- Add _sanitize_env_lines() that splits concatenated known keys and drops
*** placeholders
- Add sanitize_env_file() public API for explicit repair
- Call sanitization in save_env_value() on every read (self-healing)
- Call sanitize_env_file() at the start of migrate_config() so existing
corrupted files are repaired on update
- 12 new tests covering splits, placeholders, edge cases, and integration
634c1f6752