* docs: add DCO, governance model, roles, vulnerability credit policy
* security: migrate token hashing from SHA256 to Argon2id
- Replace unsalted SHA256 with Argon2id (salted) for API token hashing
- Fix TOCTOU race: replace exists()+read() with read()+match on error
- Set chmod 600 on token files and 700 on token storage directory
- Auto-migrate legacy SHA256 tokens to Argon2id on first verification
- Add regression tests: argon2 format, legacy migration, file permissions
