docs(mcp): add terminal-state workflow

[JJasonSun]

Summary

• document the asynchronous terminal-state workflow for MCP run_call
• persist the exact run_id and resume get_call_run after a local timeout, disconnect, or restart
• clarify that the suggested polling cadence controls query timing and is not a completion deadline
• document terminal statuses and clarify that COMPLETED still requires application-level result evaluation
• follow next_step for undocumented statuses and uncertain outcomes; when no run_id is returned, use an explicit recovery path or operator review instead of automatic resubmission
• distinguish MCP status polling from the Developer API webhook flow

Validation

• pnpm test
• pnpm check
• pnpm pack:dry-run
• git diff --check

Closes CALLE-AI/awesome-phone-call-agents#160