docs(pm-dispatch): the Writes: line names the relay as the only GitHub write path and bans GitHub MCP writes by name; the card thread's opening message carries it verbatim; contract review notes a PR not authored by the fleet bot

docs(pm-dispatch): the Writes: line names the relay as the only GitHub write path and bans GitHub MCP writes by name; the card thread's opening message carries it verbatim; contract review notes a PR not authored by the fleet bot #21176