docs(protocol): mandate guarded_write / verify_file_lock before overwriting

Closes the participating-agent half of "enforced locks": agents that follow the
protocol must route writes through guarded_write (enforced) or call
verify_file_lock immediately before editing (detection), and never force a write
on CONFLICT/DENIED. A process that ignores Axis entirely cannot be prevented at
the MCP layer — that is an architectural limit, documented as such.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>