fix: add drain timeout margin

[chubes4]

Summary

• Adds --stop-before-timeout to wp datamachine drain so local drains can exit before an external timeout kills the process.
• Reports stop_reason from drain runs, including timeout_margin, so JSON/table output explains why the drain stopped.
• Updates WP-CLI docs and smoke coverage for the new option.

Verification

• php -l inc/Cli/Commands/DrainCommand.php
• php tests/flow-run-cli-drain-smoke.php

Linked issues

• Fixes Add stop-before-timeout support to local drain command data-machine-code#445

Notes

• homeboy lint --path /Users/chubes/Developer/data-machine@fix-drain-stop-before-timeout --extension wordpress could not run because the local Homeboy extension registry does not have the wordpress extension linked, even though this component declares it.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Diagnosing the CLI gap, drafting the implementation, updating smoke coverage/docs, and running local verification. Chris remains responsible for review and merge.