[codex] Skip private writing paths

[dbtlr]

Summary

Updates the writing sync process so underscore-prefixed source files and directories are treated as private authoring/process space.

What changed

• scripts/sync-writing.ts now skips any directory entry whose basename starts with _ during traversal.
• README documents that _templates/, _meta.md, and similar vault-side publishing/process paths are ignored entirely.

Why

This lets the local writing source folder contain templates, metadata, notes, and publishing process scaffolding beside article drafts without the sync script parsing or reporting them.

Validation

• Fixture sync test where _meta.md and _templates/template.md looked publishable but were ignored, while articles/real-post.md synced.
• pnpm sync:writing -- --dry-run
• 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]

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_{Updated for 269e54c · ephemeral version, not production.}