Follow up on no_ostree build tag: README docs and CI coverage

Follow-up to #73, which added the no_ostree build tag (go build -tags no_ostree / make build-no-ostree) so oci-delta can be built without the libostree/ostree-devel dependency.

Two gaps to close:

1. README/docs update — the no_ostree build mode isn't documented anywhere user-facing (README.md, CONTRIBUTING.md, man page). Add a short section explaining what it does, how to build with it, and what functionality is unavailable (apply --ostree-repo).

2. CI coverage — .github/workflows/ci.yml doesn't build or vet with -tags no_ostree. Since the tag relies on ostreerepo_no_ostree.go staying in sync with the DataSource interface and OstreeRepoDataSource's method set, an untested build path can silently break if either changes. Add a CI step (or job) that runs:

   • go build -tags no_ostree ./...
   • go vet -tags no_ostree ./...

Start with README.md, CONTRIBUTING.md, the man page, and .github/workflows/ci.yml, then review the existing no_ostree build-tag files and the commands from issue #73. Document the build mode, its unavailable ostree-repo functionality, and add CI coverage for the specified build and vet commands. Done means the documentation is user-facing and both no_ostree checks pass in CI.