docs: show when each page last changed

[EivMeyer]

Summary

Every guide page now ends with "Last updated" and a date, so readers can tell how current it is. The date is the last commit that changed that page's source. The Zeroshot docs get the same line in the-open-engine/zeroshot#1224.

Details

• scripts/build-docs.py reads the date from Git and appends the line to each page before rendering.
• The CLI reference has no source file of its own; it is dated by src/cli_args.rs, which it is generated from. Rustdoc pages and directory listings are not dated.
• A shallow clone credits every file to its one visible commit, so a build without full history shows no date instead of a wrong one. docs.yml now fetches full history for the source checkout; the pull-request build is shallow and shows none.
• As with any docs change, this reaches dev on merge and stable with the next release.

Verification

• scripts/build-docs.sh on a full checkout: 61 guide pages carry a date, each its own (overview 4 October, getting started 24 September, CLI 22 September)
• New test in test_docs_versions.py: the newest commit touching a file wins, and an untracked file, a shallow clone and a directory outside Git give no line. 24 pass.
• opcore check against main: clean. opcore run pre-commit: Verify clean, no Sense findings; it still exits requires_attention on the repository's partial Sense coverage, as on main.
• The changed workflow file parses, with fetch-depth: 0 on the source checkout

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