docs(site): document Pi and subagent model configuration

[rochecompaan]

Summary

• Added a dedicated public-site guide for Patchmill Pi orchestrator and built-in subagent model/thinking configuration.
• Linked the guide from configuration, provider, and workflow-lifecycle documentation.
• Added committed visual evidence for the rendered guide route.

Validation

• npm run lint:md — passed, Summary: 0 error(s)
• npm run site:build — passed, 17 page(s) built including /guides/pi-and-subagents/
• git diff --check 75283f5..HEAD — passed, no output
• git status --short — clean
• docs/screenshots/pi-and-subagents.png — committed PNG reference screenshot

Reviews

• Final Codex full-worktree review after visual evidence and caution-block fix — pass, no findings
• Final thermo-nuclear full-worktree review after visual evidence and caution-block fix — pass, no findings
• Final validation-readiness review — pass, all required commands passed

Closes #106