Show last-built timestamp + linked commit SHA in footer

[sroertgen]

Summary

Adds Last built: YYYY-MM-DD HH:mm UTC (sha) to the footer, where the short SHA links to the commit on the configured repository host. Closes #341.

The motivation in the issue: CI logs sometimes report Error even on successful builds, so users (and the team) need a quick way to verify whether the deployed site reflects the latest source.

Implementation

• src/buildInfo.js — pure helpers (formatBuildTime, shortSha, commitUrl), browser-safe, imported by the footer
• src/resolveGitCommit.js — Node-only helper (kept separate so child_process doesn't get pulled into the browser bundle); resolves the SHA from GITHUB_SHA → CI_COMMIT_SHA → git rev-parse HEAD → null. Uses dependency injection for execSync to keep tests fast and free of CJS/ESM mocking gymnastics.
• gatsby-config.js — exposes gitCommit + repositoryUrl on siteMetadata (coerced to "" when missing, matching the existing customDomain: "" convention so Gatsby's automatic schema inference stays happy)
• src/hooks/configAndConceptSchemes.js — also queries Gatsby's built-in site.buildTime
• src/components/footer.jsx — renders four conditional shapes: Source link only, Source + timestamp, Source + timestamp + plain SHA (no repo URL), Source + timestamp + linked SHA. The footer no longer reads process.env.GATSBY_RESPOSITORY_URL directly — it gets repositoryUrl from the hook.

Note: the existing typo GATSBY_RESPOSITORY_URL (should be REPOSITORY) is preserved to avoid a breaking config change for existing deployments.