Link the About page's commit hash to the commit

Branch feature/commit-link · version 0.5.3

What changed

Linked hash: the About page's build hash, e.g. 0.5.3 (d2a387e), now links to that commit in Gogs. The link uses the full hash: https://git.monoexp.com/bajdev/posts/commit/<full hash>.
New repoURL setting: optional, in content/site.json, defaulting to https://git.monoexp.com/bajdev/posts. Anyone running a fork of the code can point the link at their own repo.
Dirty builds aren't linked. A binary built from uncommitted changes matches no commit, so it still shows (abc1234-dirty) as plain text. Pipeline builds always come from a clean checkout, so the live site always has the link.
README: documents repoURL.
Version: 0.5.3.

Since Phase 7, the live site is always built from the tip of main, so the link goes to the merge commit that's running.
The hash display is unchanged; only the link is new.

All tests pass with the race detector (go test -race ./...).
New test covering four cases: a clean build links the full hash, repoURL in site.json overrides the link, a dirty build shows -dirty with no link, and no commit at all shows the bare version.
Built from a clean local commit and checked the About page HTML: the link went to /commit/<full hash> of that build. Gogs commit URLs return 200 for the full hash.

About page

From the branch's build running locally. The home page doesn't change.

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