Agent-readable docs: Phase 2 — llms.txt and frontmatter descriptions

[tbantle22]

Phase 2 of notes/agent-readable-docs.md, building on the Markdown alternates from Phase 1 (PR #101). Adds a hierarchical, agent-readable index of every docs page (llms.txt, per llmstxt.org) and starts wiring frontmatter description: into the entries.

What's in

File(s)	Change
site/{dolt,doltlab,doltgres}/src/pages/llms.txt.ts (new)	Static endpoint that emits llms.txt: H1 with the site title, blockquote summary, one H2 per top-level nav section, nested bullets for each page linking to the .md alternate. Descriptions come from the page's frontmatter description: field where present. Order matches nav.ts (so the sidebar).
site/{dolt,doltlab,doltgres}/_headers	Explicit Content-Type: text/markdown for /docs/llms.txt and the bundle-root /llms.txt. Cloudflare otherwise ships text/plain.
site/{dolt,doltlab,doltgres}/package.json	cp dist/docs/llms.txt dist/llms.txt in each build script, next to the existing _headers / _redirects / 404.html copies.
site/{dolt,doltlab,doltgres}/src/pages/404.astro	Surface the llms.txt link as another discovery path from the helpful-404.
Frontmatter description: on a sample of high-traffic pages	Dolt: introduction (what-is-dolt, installation, getting-started), the three top git concept pages (commits, branch, merge), products (hosted overview + cloning, dolthub overview), and the three big reference landings (CLI, dolt-sql-procedures, dolt-system-tables). DoltLab: what-is-doltlab + the two admin guides. Doltgres: introduction overview, installation, getting-started. Pages without a description still appear in llms.txt — they just don't get the descriptive suffix on the entry.
cypress/e2e/dolt/llms-txt.spec.ts (new)	Verifies the file is served at /docs/llms.txt, starts with the title + blockquote summary, has one H2 per nav section, every entry links to a .md URL, and frontmatter descriptions appear after the colon.

Verification

• Dolt llms.txt: 139 entries across 8 sections (167 lines). DoltLab: 97 lines. Doltgres: 77 lines.
• Build emits the file at both dist/docs/llms.txt and dist/llms.txt for each site.
• Cypress: llms-txt (4/4), markdown-alternate (4/4), navigation (5/5), dead-links (15/15), single-h1 (10/10) — all pass.

What's still ahead

Phase 3 (breadcrumbs, related-docs footer, broader description: backfill) per notes/agent-readable-docs.md. Phase 2 leaves us with a clean discovery surface even before that lands.