feat(gnoweb): serve realm pages as markdown via Accept negotiation

[gfanton]

Summary

gnoweb now serves a realm page (and static-markdown aliases) as raw text/markdown
when the client asks for it through the Accept header. The bytes are the exact
output of the realm's Render() function, written without the HTML page layout and
without the goldmark render step. Every other request is unchanged and still gets HTML.

Motivation

Realm Render() already returns markdown; gnoweb converts it to HTML for the browser,
and clients that prefer the source (LLM agents, scripts, doc tooling) had no way to get
it. For example, Claude Code's WebFetch sends Accept: text/markdown, text/html, */*,
so it now receives the realm markdown directly (about 4.7 KB for the home realm) instead
of the full HTML page (about 58 KB), with no manual configuration.

How it works

• Negotiation is Accept-only: a response is markdown when the header names
  text/markdown (or the alias text/x-markdown) with a non-zero q-value. It never
  matches via */*, so browsers keep receiving HTML. An explicit q=0 is honored as a refusal.
• For a realm, the raw Render() bytes are written verbatim with
  Content-Type: text/markdown; charset=utf-8, bypassing IndexLayout; the goldmark
  parse is skipped on this path.
• Vary: Accept is set on GET responses so shared caches key markdown and HTML
  separately for the same URL.
• Scope is realm pages and static-markdown aliases. Source, help, directory and user
  views fall back to HTML for now (follow-up work).

Test plan

• negotiate_test.go: table covering the header rules (aliases, q-values, */* and
  text/* wildcards, q=0 refusal, the real WebFetch Accept string).
• view_markdown_test.go: verbatim render of the markdown view.
• handler_http_test.go: end-to-end through ServeHTTP, asserting Content-Type,
  Vary, markdown vs HTML bodies, and that a realm with no Render() still falls back to HTML.
• Verified live against gnodev:
  curl -H 'Accept: text/markdown' http://127.0.0.1:8888/r/gnoland/home returns markdown;
  a plain request returns the HTML page.

[Gno2D2]

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[davd-gzl]

Clean and well-tested; verified on the current head (4ab2753). Nothing blocking.

Full review: https://github.com/samouraiworld/gno-agent-workspace/blob/main/reviews/pr/5xxx/5794-markdown-accept-negotiation/2-4ab275316/claude-opus-4-8_davd-gzl.md

(AI Agent)

---

Code LGTM!

[davd-gzl]

text/html;q=0.9, text/markdown;q=0.8 returns markdown: any non-zero q wins regardless of client ranking, a shortcut over RFC 9110 preference ordering. Harmless since browsers never send text/markdown; confirming it's intentional.

repro

# from a local clone of gnolang/gno:
gh pr checkout 5794 -R gnolang/gno
go test ./gno.land/pkg/gnoweb/ -run 'TestNegotiatesMarkdown/markdown_present_with_non-zero_q_among_others' -v

=== RUN   TestNegotiatesMarkdown/markdown_present_with_non-zero_q_among_others
--- PASS: TestNegotiatesMarkdown/markdown_present_with_non-zero_q_among_others (0.00s)
PASS

negotiate_test.go:31 asserts want: true for this header.

(AI Agent)

[davd-gzl]

This write serves realm Render() bytes verbatim (the HTML path sanitizes through goldmark) and gnoweb sets no nosniff anywhere. Add X-Content-Type-Options: nosniff here.

(AI Agent)