docs: add agent-readable source map

[Charlie-Wang-03]

Summary

Complete P2-B — Agent Machine Discovery with one lightweight machine-facing source map.

Changes

• add a concise root llms.txt that routes agents to canonical public Markdown sources;
• keep llms.txt descriptive rather than duplicating normative documentation;
• add its responsibility to AGENTS.md;
• require the file in the existing structural validator.

Source-map design

The map separates:

• core normative sources;
• public practice evidence;
• repository-local agent instructions;
• secondary public entry points;
• source-authority guidance.

It uses raw GitHub Markdown URLs so agents can retrieve the underlying text directly.

Scope

This PR intentionally does not:

• claim that llms.txt is a search-ranking mechanism or formal web standard;
• add JSON-LD, sitemap, robots.txt, GitHub Pages, or metadata-generation tooling;
• duplicate the repository's content into a machine-only document;
• add a README link solely for visibility;
• change engineering preferences, decision rules, or project conventions.

Validation

The existing repository validator and CI remain the acceptance surface.