Publish per-page Markdown twins at {route}.md instead of {route}/index.md

[phil-scott-78]

What

Per-page LLM Markdown twins now land at {route}.md (Claude-docs style) instead of {route}/index.md.

The page /getting-started/first-page now publishes its Markdown copy at /getting-started/first-page.md — a sibling of the first-page/ page folder — rather than nesting it inside as /getting-started/first-page/index.md. This conforms to how Claude's docs and the broader llms.txt ecosystem name their Markdown copies.

The root home stays at /index.md (since /.md isn't a usable URL, and the index.llms.md machine-home already keys on /index.md).

Why

{route}/index.md forced the agent-facing copy into a nested folder. {route}.md is the convention agents (and Claude Code's Accept: text/markdown WebFetch) expect, and it keeps the twin next to the page in the output tree.

Changes

Core

• LlmsTxtService.BuildCoLocatedMarkdownPath emits {key}.md (root index.md). This is the single source the public-URL builder, internal-link rewriter, and HTML <link rel="alternate"> hints all derive from.
• LlmsArtifactContentService collapses the ExactClaim("/index.md") + SuffixClaim("/index.md") pair into one SuffixClaim(".md") — covers root and every nested twin. Content routes are extensionless so it shadows none, and the resolver stays authoritative (returns null → router falls through) for any .md it didn't generate.

Serving / chrome

• DocSite Pages.razor + App.razor body cue, BlogSite Blog.razor alternate hints.
• docs/cloudflare/_worker.js — markdownTwin() now maps /x → /x.md, /x/ and /x/index.html → /x.md, and / → /index.md for production Accept: text/markdown negotiation.

Tests & docs

• Updated LlmsTxtAndSearchEndpointTests assertions, plus the docs/examples that describe the convention (index.llms.md, the markdown-for-agents tutorial, the llms-txt how-to, two example sites).

Verification

• dotnet build clean.
• LlmsTxtAndSearchEndpointTests (17) + projection/smoke tests (13) pass.
• A real dotnet run -- build emits 521 {route}.md twins and exactly one index.md (the root). Confirmed tutorials/getting-started/first-page.md coexists with first-page/, the llms-only migrating-via-ai.md has no HTML folder, and sidecar_url points at .md while canonical_url keeps the page's trailing-slash URL.

Note for reviewers

docs/Pennington.Docs/Content/blog/llms-txt.md is intentionally not in this PR — its index.md → .md correction is entangled with separate in-flight blog-revamp edits in the working tree, and will ride along with that work.

[github-actions]

🛰️ Docs preview: https://pr-56.pennington-dev.pages.dev

Rebuilt on every push to this PR; torn down when it closes.