Use page-level dates in sitemap

[portdeveloper]

What changed

• Tracks each sitemap route's source page file.
• Uses the page file's latest Git commit date for lastmod.
• Falls back to filesystem mtime when Git metadata is unavailable.
• Batches Git history lookup so build time stays close to the existing baseline.

Why

A single build-wide lastmod date is too coarse for crawlers and AI/RAG ingestion jobs. Page-level freshness helps downstream consumers prioritize only changed docs.

Fixes #166

Merge order

Independent. This PR can be merged in any order relative to #164 and #165.

Validation

• pnpm build
• Checked generated docs/dist/sitemap.xml: 46 URLs, 5 unique lastmod values in this checkout.

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