feat: embed Markdown User Guide and AGENTS.md in agent-facing distributions

[osvaldomcc]

Description

• Add a job step to the framework distribution workflow that builds the User Guide as Markdown with sphinx-markdown-builder and embeds it into system/Docs, so coding agents get version-matched documentation alongside the distributed framework.
• Ship �dmin/framework/AGENTS.md to the appstarter repository root on release, pointing agents to the embedded docs.

No framework runtime code is touched; changes are limited to GitHub Actions workflow, the deploy script, and Sphinx build requirements.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication) — no PHP code changed
• Unit testing, with >80% coverage — no PHP code changed; covered by the workflow's est -f ./source/system/Docs/index.md verification step
• User guide updated — no public API/behavior change; CI and release tooling only
• Conforms to style guide