Documentation Site, AGENTS.md improvements and CI

[sid-ak]

Summary

• Add a Sphinx + MyST + Furo documentation site: the existing docs/ narrative tree plus an
  autodoc API reference for rag_core, deployed to GitHub Pages by a dedicated docs-deploy workflow. (3e63f96)
• Remove the docs-sync automation (the docs/issues.md and docs/checkpoints.md mirrors, their
  workflows and scripts) as redundant with GitHub PRs, commits, and issues. (7b87e9b)
• Standardize the root and rag_core AGENTS.md to the agents.md convention and strip duplication
  of architecture, ADR, and status content that lives elsewhere. (2d7da85)
• CI: add a build-only docs job, scope the push trigger to main/dev to stop duplicate
  feature-branch runs, and bump actions to Node 24 (setup-uv@v7, Pages actions @v5). (5cff232)

Motivation

• One always-current site for the narrative docs and the typed API, instead of hand-maintained,
  drift-prone references.
• Drop automation whose output duplicates GitHub's own PRs/commits/issues and needed constant
  writes to dev.
• Keep CI lean and free of the Node 20 deprecation warning.

Type

• Bug
• Feature
• Improvement

Related issues

• Documentation Site #34

Verification

• uv run ruff check . && uv run ruff format --check . && uv run mypy && uv run pytest — green.
• uv run sphinx-build -b html docs site -W — builds clean (also runs as the CI docs job).
• Pages source must be set to "GitHub Actions" so docs-deploy is the sole publisher.

Decision records / ADRs

• ADR 0005 — Documentation site tooling: Sphinx, MyST, and Furo (over MkDocs and the other options).