feat: embed agent-ready docs and AGENTS.md in framework distributions

[osvaldomcc]

Make the CodeIgniter 4 distribution friendlier to AI coding agents (Codex,
Claude Code, Cursor, Copilot, etc.) by shipping managed agent instructions and
version-matched documentation in the artifacts end users download, following
the same AGENTS.md convention adopted by other frameworks.

1. Ship AGENTS.md in both distributable repos

• Add admin/framework/AGENTS.md with managed agent instructions that direct
  agents to the bundled documentation (system/Docs for the framework
  distribution, vendor/codeigniter4/framework/system/Docs for Composer /
  app starter installs).
• Extend .github/scripts/deploy-appstarter to copy the same canonical file
  into the app starter root, so a single source serves both install channels
  (manual zip download and composer create-project).

2. Embed the user guide as Markdown in system/Docs

• The framework release job in deploy-distributables.yml now converts the
  Sphinx user guide to Markdown with sphinx-markdown-builder and embeds it
  into system/Docs before the distribution is assembled.
• _static, changelogs and images are excluded from this build (the
  output is LLM-oriented); the HTML user guide build is unaffected since the
  exclusions are passed only to the Markdown invocation via -D.
• A test -f system/Docs/index.md guard fails the job if the conversion
  produces no output, so a release can never ship without its embedded docs.

No framework runtime code is touched: no BC impact and nothing applicable for
PHP unit tests or PHPDoc (CI/distribution changes only). Docs ship inside the
release tag, so agents always get documentation matching the installed
framework version.

[carson-codeigniter4]

Hi there, @osvaldomcc! 👋

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---

Hi there, @osvaldomcc! 👋

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