feat(site): publish llms.txt and llms-full.txt for agents that browse

[TMHSDigital]

Closes #384.

Change

• scripts/site/build_site.py: new write_llms_txt(), called after write_sitemap(). It writes two files.
  • llms.txt (llmstxt.org shape) contains:
    • an H1 and a quoted summary (site.json description);
    • the install lines;
    • ## Skills (16) and ## Rules (9), each linked to the raw file on main with its full frontmatter description. These now carry the "Use when…" triggers from skills: descriptions have no 'Use when…' trigger phrasing, which hurts auto-selection in Claude Code and Cursor #357;
    • ## Examples and ## Optional (snippets, templates, the Claude rules summary).
  • llms-full.txt: every SKILL.md body plus claude/blender-rules.md. That's about 169 KB, for agents that want the whole pack in one fetch.
• Landing <head> gets <link rel="alternate" type="text/plain" href="llms.txt">. Because check_site_links.py resolves <link href>, the gate fails if the build ever stops writing the file.
• .gitignore: both files are build outputs, like index.html and sitemap.xml. stage_public.py keeps them, since it only drops top-level .md files.

Evidence (local)

• build_site.py → build_gallery.py → stage_public.py → check_site_links.py --root _site: 0 failures. The staged tree contains llms.txt (14.5 KB) and llms-full.txt (169 KB).
• Falsified: deleting llms.txt from the staged tree makes the link gate report it missing.
• Spot-checked lines: - [addon-scaffolding](https://raw.githubusercontent.com/…/skills/addon-scaffolding/SKILL.md): Scaffold a Blender add-on … and the rule line for prefer-temp-override-over-context-copy.

🤖 Generated with Claude Code

[TMHSDigital]

CI evidence before merge: all checks pass ( 13 pass ). Blender Smoke on PR head: Blender 5.2.2 LTS / Blender 4.5.14 LTS. Summaries (happy path; falsifiers) per leg: 143 passed, 1 skipped, 0 failed;143 passed, 0 skipped, 0 failed;141 passed, 3 skipped, 0 failed;136 passed, 7 skipped, 0 failed;