Add automated docs sync pipeline from forge repo

[initializ-mk]

Summary

• Add scripts/sync-docs.ts that fetches docs from initializ/forge via GitHub API, parses YAML frontmatter, rewrites relative links to Astro routes, and generates docs-manifest.json
• Add scripts/sync-docs-stub.ts for generating placeholder docs during local dev without GitHub access
• Remove manually-maintained docs from git — they are now generated at build time via npm run sync:docs

How it works

1. Forge repo has restructured docs in subdirectories (getting-started/, core-concepts/, etc.) with YAML frontmatter — see forge PR branch
2. sync-docs.ts fetches the git tree, downloads each .md, validates frontmatter, rewrites links, and writes to src/content/docs/
3. sync:docs runs first in the fetch pipeline so docs exist before Astro processes other pages
4. Stub script generates minimal placeholders for astro dev without network access

Changes

• scripts/sync-docs.ts — main sync script (fetches, parses, rewrites, writes)
• scripts/sync-docs-stub.ts — dev stub generator (36 placeholder pages)
• package.json — add sync:docs, sync:docs:stub; sync:docs runs first in fetch
• .gitignore — exclude synced docs directories and manifest
• Removed 36 manually-maintained doc files from git tracking

Test plan

• npx tsx scripts/sync-docs-stub.ts generates 36 placeholder docs
• npx tsx scripts/sync-docs.ts fetches docs (once forge branch is merged)
• npm run build succeeds with synced/stub docs
• Synced files are in .gitignore, not tracked in git
• docs-manifest.json generated with correct section counts