Add a methodology-only llm endpoint (phases 1–2 without the XLSForm spec)

[jstet]

formulaid (the survey generator, CorrelAid/formulaid) wants this site's survey methodology (phases 1–2) without the XLSForm specification. For question types it already ships the registry-generated cdl-survey-types reference from formtransform, which is authoritative, and the general spec in its context contradicts it.

Today scripts/build_skill.sh in formulaid fetches https://umfragen.civic-data.de/llm-phases12-xlsform.txt (LLM_METHODOLOGY_URL, line 26). It then deletes everything between the headings # XLSForm-Dokumentation and # Fragen formulieren (lines 185–192), and checks for Fragen formulieren again at line 268. That breaks silently if those headings change. A dedicated endpoint removes the guesswork.

Steps

1. Add src/pages/llm-phases12.txt.ts, the same wrapper as the existing llm-*.txt.ts files, calling buildLlmTxt('none', 2): phases 1–2 without XLSForm examples and without the xlsform-standard page.
2. buildLlmTxt / getSlugsForPhases (src/lib/utils/llm-txt.ts, lines 247 and 232) need a way to leave a page out of text output. toc.json already marks xlsform-standard in excludeFromGraphic for the Fragebogendesign section (line 25). Add an equivalent text exclusion (e.g. excludeFromText), or have the 'none' format skip it. Don't reuse excludeFromGraphic for this; the two may diverge.
3. Keep every existing endpoint working and unchanged. Other consumers may rely on them.
4. Once it's live, open an issue on CorrelAid/formulaid: switch LLM_METHODOLOGY_URL to /llm-phases12.txt and drop the heading strip. Link it here.

Acceptance criteria

• curl localhost:4321/llm-phases12.txt (under bun run dev) contains the Datenschutz and Operationalisierung sections and no # XLSForm-Dokumentation heading
• llm-phases12-xlsform.txt, llm-xlsform.txt, llm.txt, llm-ddi.txt are byte-identical before/after (diff the built files)
• formulaid issue filed and linked
• bun run build passes

Until formulaid has switched, don't rename # XLSForm-Dokumentation or # Fragen formulieren.

Rules (apply to all site changes around formtransform)

• Don't restate the supported XLSForm subset in prose. Link to it: https://github.com/CorrelAid/formtransform#supported-xlsform-subset. A hand-copied type list on the site becomes a third source of truth, after the registry and the generated skill.

• Don't rename or delete these snippet paths. Three apps fetch them at build time through the GitHub API, and each falls back to an empty string, so a rename doesn't fail a build. It silently empties a section of the app:

  App	Fetches
  formtransform-app	src/content/snippets/formtransform/{en,de}.html, liability/{en,de}.html
  qwac	qwac/{en,de}.html, liability/{en,de}.html
  formulaid	formulaid/{en,de}.html, liability/{en,de}.html

• Headings in the llm*.txt endpoints are an interface. formulaid's scripts/build_skill.sh fetches llm-phases12-xlsform.txt and slices it by the headings # XLSForm-Dokumentation and # Fragen formulieren. Don't change them without telling CorrelAid/formulaid.

• German is primary; EN/DE snippets stay in sync. The apps render whichever locale the visitor has.

• Registry facts belong to CorrelAid/formtransform. If the site needs a machine-readable list of supported types, open an issue there. Don't copy JSON into the site.