Add llms.txt to docs for AI agent discovery

[gregology]

Closes #40

AI agents and tools that support the llmstxt.org spec can now discover sctx docs automatically at sctx.dev/llms.txt.

The file follows the standard format: H1 title, blockquote summary, a short description paragraph, then H2 sections linking out to the existing docs pages (getting started, protocol spec, CLI reference, examples, comparisons, contributing, roadmap).

MkDocs already copies non-.md files straight through to the site output, so no config changes needed — just dropping the file into docs/ is enough.

I considered adding it as a markdown file with a custom template to keep everything in one format, but that's overcomplicating it. The whole point of llms.txt is that it's a plain text file with light markdown structure. Keeping it as a .txt also means it gets served with the right content type and there's zero chance of MkDocs wrapping it in the site theme.

bot metrics

Total duration: 3m 52s
Turns: 69
Tool calls: 60
Tokens: 326,486 input / 5,196 output

Stage	Model	Duration	Turns	Tool calls	Tokens (in/out)	Cache read	Cache creation
triage	claude-opus-4-6	2m 18s	46	43	98,706 / 2,158	92,424	6,185
implementation	claude-opus-4-6	0m 41s	10	7	82,161 / 1,708	78,006	4,143
evaluation	claude-opus-4-6	0m 8s	1	1	23,150 / 177	17,799	5,348
docs_review	claude-opus-4-6	0m 30s	11	8	90,946 / 813	82,240	8,697
craft_pr	claude-opus-4-6	0m 13s	1	1	31,523 / 340	15,582	15,938

Resolves #40