fix(sitemap): derive per-url lastModified from build metadata (#46)

[bhushan4work]

What and why

• Generate per-URL sitemap lastModified values from build-time metadata instead of sharing one build timestamp.
• Handle shallow CI checkouts without assigning every URL the same latest commit date.
• Leave guide URLs out until Build a guides content system (routes, metadata, JSON-LD, OG, sitemap) #35 is merged.

Closes #46

How I tested this

• npm run lint
• npm run test
• npm run build

AI assistance

none

Type of change

• Bug fix
• New tool or feature
• Tests
• Refactor or cleanup
• Docs

Checklist

• I was assigned the linked issue
• npm run lint && npm run test && npm run build passes locally
• Logic changes live in lib/ and have a test in lib/__tests__/
• For a bug fix: the new test fails without my change
• No analytics, trackers, or calls that send user data off-device
• Works in both light and dark themes (if UI changed)
• Inputs have labels and it works with the keyboard alone (if UI changed)
• New tool: registry entry, page.tsx, client component, opengraph-image.tsx, and the bug report dropdown are all done

Notes for reviewers

Per-URL freshness is resolved at build time; no runtime git access is required. Guides remain out of scope pending #35.

[anurag629]

Thanks @bhushan4work. On top of current develop lint, all 334 tests and the build pass, and the shallow clone handling works: a depth 1 clone unshallows and gets the same dates as a full one, and with an unreachable remote everything falls back to BUILD_DATE.

One thing to fix: / gets the date of app/page.tsx, which is 2026-07-08 in the build. But the homepage lists every tool from lib/tools.ts, so it changed on Oct 17 and Oct 19 when the color and timestamp converters were added. Can / use the newer of app/page.tsx and lib/tools.ts? git log -1 --format=%cs -- app/page.tsx lib/tools.ts gives that in one call.

And can you drop the fetch-depth: 0 from ci.yml? next.config.js already unshallows a shallow clone, and CI doesn't check the dates, so it only makes every CI run fetch the full history.

[anurag629]

Thanks @bhushan4work, both are in. On top of current develop (with #201) lint, all 360 tests, the OpenNext build and the pages pass. The built sitemap now gives / 2026-10-19 from lib/tools.ts, and each tool its own folder's date, from 2026-07-08 for the invoice generator to today for the two newest. Merging.