Integration test: gitops diff

[cicalese]

Test canasta gitops diff with local changes, remote changes, and conflicts.

The existing gitops test covers init/add/push but not diff, which has its own git fetch, local/remote diff computation, conflict detection, change analysis, and hint output. The classifyChanges helper has unit tests but the full CLI path through runDiff() is untested.

Estimated coverage gain: ~63 new statements in cmd/gitops/diff.go.

Partial progress on #241.

[cicalese]

Done in #734.