[codex] Add writing sync workflow

[dbtlr]

Summary

Adds a local writing sync workflow for publishing blog posts authored outside the repository into Astro's writing content collection.

What changed

• Added pnpm sync:writing, backed by scripts/sync-writing.ts.
• Loads DBTLR_WRITING_SOURCE_DIR from local .env via dotenv; .env is gitignored.
• Syncs only status: published Markdown files and skips drafts/stubs.
• Generates Astro-compatible Markdown in src/content/writing/.
• Tracks stable source-to-slug mappings in src/content/writing/.sync-manifest.json using source-relative filenames, not absolute local paths.
• Supports --dry-run, --prune, and --rename-slugs.
• Adds synced writing posts to the build-time search index.
• Documents the workflow in the README.

Validation

• pnpm sync:writing -- --dry-run
• Fixture tests for draft skip, create/update, stable title-change slugs, prune, and opt-in slug rename
• pnpm exec tsc --ignoreConfig --noEmit --module NodeNext --moduleResolution NodeNext --target ES2024 --types node --esModuleInterop scripts/sync-writing.ts
• pnpm check
• pnpm build

pnpm check and pnpm build still show the known empty-writing collection warning until the first article exists.

[github-actions]

🔎 Preview deploy

https://52231f28-dbtlr-com.dbs-account.workers.dev

_{Updated for 7fce9f2 · ephemeral version, not production.}