[Doc Drift] README extract flags table missing `--remove-stale`

### What drifted

The `apiops extract` flags table in `README.md` is missing the `--remove-stale` flag. The flag exists in the CLI (confirmed by `apiops extract --help`) and is documented in `docs/commands/extract.md`, but was not added to the README summary table when it was introduced in commit `570ca00` ("fix: reconcile stale extraction artifacts safely").

### Source of truth

`apiops extract --help` output:
```
--remove-stale Remove stale managed artifacts in the selected scope
```

`docs/commands/extract.md` line 69:
```
| `--remove-stale` | boolean | false | No | Remove stale managed artifacts in the selected filter and workspace scope after a fully successful extraction |
```

### Affected documentation

- `README.md` — the `### \`apiops extract\`` section's flags table (currently lists only `--resource-group`, `--service-name`, `--output`, `--filter`, `--no-transitive`)

### Suggested update

Add a row for `--remove-stale` to the README extract flags table:

| `--remove-stale` | | Remove stale managed artifacts after a fully successful extraction |

> Generated by [Documentation Freshness Check — System Policy](https://github.com/Azure/apiops-cli/actions/runs/33410933190) · 38.1 AIC · ⌖ 6.97 AIC · ⊞ 6.2K · [◷](https://github.com/search?q=repo%3AAzure%2Fapiops-cli+is%3Aissue+%22gh-aw-workflow-call-id%3A+Azure%2Fapiops-cli%2Fdoc-freshness%22&type=issues)

Start with the `apiops extract` flags table in `README.md`, then compare it with `docs/commands/extract.md` line 69 and the `apiops extract --help` output mentioned in the issue. Done means the README table includes `--remove-stale` with wording consistent with the documented behavior.

索引モデルが issue の本文から書いたものです。