ci: run Test Workspace only on main, not on pull requests (#212)

The full workspace + failpoints suite was the slowest PR gate (~15min
warm, up to the 75min cold ceiling) and dominated PR turnaround. Gate the
`test` job with `if: github.event_name != 'pull_request'` so it runs only
on push to `main` (post-merge), on `v*` tags, and on manual
`workflow_dispatch`. `RustFS S3 Integration` needs `test`, so it becomes
push-/dispatch-only by the same cascade.

Drop `Test Workspace` from the required-check list in
branch-protection.json: a required context that never reports on PRs (the
job no longer runs there) would leave every PR permanently pending — the
job-never-reports trap the policy already documents.

Trade-off accepted deliberately (chosen by the maintainer): a regression
the suite would catch now lands on `main` and reddens the post-merge run
instead of being blocked pre-merge, so `main` can briefly break. Mitigations
documented in ci.md: run `cargo test --workspace --locked` locally before
merging non-trivial changes (or trigger the workflow on your branch via
workflow_dispatch), and regenerate openapi.json locally for server/API
changes (the auto-regen step lived in the now-PR-skipped test job).

The fast PR gates remain: Classify Changes, Check AGENTS.md Links, the
AWS-feature build/test, and the two CODEOWNERS checks.

NOTE: an admin must run ./scripts/apply-branch-protection.sh after this
merges, or GitHub keeps requiring the now-unreported Test Workspace context.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/branch-protection.json

@@ -5,7 +5,6 @@
     "contexts": [
       "Classify Changes",
       "Check AGENTS.md Links",
-      "Test Workspace",
       "Test omnigraph-server --features aws",
       "CODEOWNERS matches source",
       "CODEOWNERS not hand-edited"