Add llms.txt adapter

[jstar0]

Summary

Fixes #12.

Adds a dependency-free llms.txt adapter so direct /llms.txt URLs can expose a site's declared agent-readable sections and links as tools.

The adapter parses the common llms.txt shape:

• H1 title
• blockquote summary
• pre-section descriptive text
• H2 sections with Markdown link lists
• ## Optional sections
• relative links resolved against the source URL

It also handles /llms-full.txt files that contain full context instead of link lists by returning a bounded get_full_context tool.

Changes

Adds link-list tools:

• list_sections
• get_section
• fetch_link

fetch_link only fetches declared links. It rejects non-HTTP(S), obvious local/private/link-local/internal targets, checks the final redirected URL, and caps returned response text at 20,000 characters.

Registers format adapters in the engine between OpenAPI and crypto/data adapters. executeTool() now follows the same adapter precedence as resolveTools(), with regression coverage for an OpenAPI/llms.txt detector overlap.

Verification

cd worker && npm run typecheck
cd worker && npm test -- test/engine_order.test.ts test/llmstxt_engine.test.ts
cd worker && npm run verify

Notes:

• Focused llmstxt adapter tests: 8 tests passed.
• Focused worker tests: 2 test files passed.
• Worker verify: 19 test files, 179 tests passed.
• The full adapter harness currently has an unrelated baseline failure in an existing JSON-LD Article expectation on main; the focused llmstxt adapter coverage passes cleanly.