feat: llms.txt should include full page content, not just TOC

Problem

`src/server/routes/llms.txt.ts` and `src/lib/llms.ts` currently generate a table-of-contents-style output with just titles and URLs. The actual page content is not included, making the `llms.txt` output insufficient for LLM consumption.

`get-llm-text.ts` does fetch processed page content for individual pages, but the index route only lists titles.

Expected Behavior

The `llms.txt` endpoint should follow the emerging `llms.txt` convention:

• `/llms.txt` — index with titles, descriptions, and URLs
• `/llms-full.txt` — full content dump of all pages (for smaller sites)
• Individual page `.md` routes already work via `[...slug].md.ts`

The index should at minimum include page descriptions alongside titles.

Start with src/server/routes/llms.txt.ts and src/lib/llms.ts, then compare get-llm-text.ts and [...slug].md.ts to understand how processed page content is obtained. Verify that /llms.txt includes titles, descriptions, and URLs, while /llms-full.txt includes all page content and individual .md routes remain working.