chore: standalone Halfstreet repo scaffolding

- package.json: drop @astrojs/sitemap, fontsource fonts, wrangler,
  @cloudflare/workers-types, tsx, spotify scripts. Keep astro/yaml/zod
  + vitest/typescript/@astrojs/check/@types/node. Add GPL-3.0-or-later.
- astro.config.mjs: drop sitemap integration, sharp image config; set
  site to halfstreet.io.
- tsconfig.json: drop cloudflare-workers-types, drop functions/scripts
  from include.
- .gitignore: cloudflare-free; Obsidian workspace cache rules updated
  to post-rename src/world/.obsidian/ paths.
- README.md: stack, layout, design-doc index, license note.
- .woodpecker.yml: install + npm test + npm run build pipeline.

.woodpecker.yml

@@ -0,0 +1,18 @@
+when:
+  - event: [push, pull_request, manual]
+
+steps:
+  - name: install
+    image: node:22-alpine
+    commands:
+      - npm ci
+
+  - name: test
+    image: node:22-alpine
+    commands:
+      - npm test
+
+  - name: build
+    image: node:22-alpine
+    commands:
+      - npm run build