Step Summary: include CLI command as a copyable code block (CLI Local Review Phase 1, step 5 of 5)

[reuvenharrison]

Final step of CLI Local Review Phase 1. The free breaking and changelog actions' Step Summary already emit a clickable link to /review?owner=.... This PR adds the CLI command itself as a fenced code block alongside the link, so visitors who recognize the command can copy it directly from the PR's Checks tab and bypass the instruction-page detour.

What changes

Before (the existing emit):

### 📋 [Review & approve these breaking changes](https://www.oasdiff.com/review?owner=...)

After:

### 📋 [Review & approve these breaking changes](https://www.oasdiff.com/review?owner=...)

Or run locally in your clone of `<repo>`:

```bash
oasdiff breaking <base_sha>:<base_file> <head_sha>:<rev_file> --open

GitHub renders the fenced code block with a built-in copy button. The familiar-visitor cohort one-steps the flow; the unfamiliar-visitor cohort still has the link and lands on the instruction page (w3 #245), which hands them the same pre-filled command with explanatory copy.

Same change for the `changelog` action (`oasdiff changelog` instead of `oasdiff breaking`).

## Backward compat

Customers running pinned older versions (e.g. `oasdiff/oasdiff-action/breaking@v0.0.48`) see the old Step Summary, which still works — the link still resolves to the instruction page after w3 #245 ships, and that page hands them the command they would have seen here. No customer is broken by skipping this upgrade.

## Test plan

- [ ] Trigger the breaking action on a test PR with a real breaking change. Check the Step Summary: link + code block both rendered, code block has GitHub's copy button.
- [ ] Copy the command and run in a local clone. Confirm it generates a working `/review/local/[id]` URL.
- [ ] Same for the changelog action.

## Depends on

oasdiff #955 (the `--open` flag the emitted command depends on) — recommended sequence: ship #955 first, then this PR.