fix(mcp): document review-pr label and issue flags

[aryansk]

Summary

• Document the repeatable --label and --issue flags accepted by review-pr.
• Explain that --linked-issue takes precedence when both issue flag forms are supplied.
• Add a package-local regression test for the rendered help output.

Scope

• The PR title follows type(scope): short summary Conventional Commit format.
• This PR is focused and does not mix unrelated backend, UI, MCP, docs, dependency, and deploy changes.
• This follows CONTRIBUTING.md and does not reintroduce GitHub Pages, VitePress, site/, or CNAME.
• I linked a currently open issue this PR resolves: Closes #10306.

Validation

• git diff --check
• npm run build:mcp
• npm run test:mcp-pack
• npx vitest run test/unit/mcp-cli-review-pr.test.ts packages/loopover-mcp/test/review-pr-help.test.ts
• npx prettier --check packages/loopover-mcp/bin/loopover-mcp.ts packages/loopover-mcp/test/review-pr-help.test.ts packages/loopover-mcp/README.md vitest.config.ts
• npm run actionlint
• npm run typecheck
• npm run test:coverage
• npm run test:workers
• npm run ui:openapi:check
• npm run ui:lint
• npm run ui:typecheck
• npm run ui:build
• npm audit --audit-level=moderate
• New behavior has a regression test for the help-text branches exercised here.

If any required check was skipped, explain why:

• The change is limited to MCP help text, README usage text, the root Vitest include wiring needed to run the package-local test, and that regression test. The full npm run test:ci chain and unrelated UI/worker checks were not run locally.
• The audit was run with npm audit --audit-level=moderate --omit=dev; it reports six existing transitive advisories in dependencies outside this docs-only change. No dependency files were modified.

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores, private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized and does not imply compensation guarantees or optimization tactics.
• No auth, cookie, CORS, GitHub App, Cloudflare, or session changes were made.
• No API/OpenAPI/MCP behavior was changed; the existing help output was documented and tested.
• No frontend UI changes were made.
• Public docs were updated where needed; no changelog was edited.

UI Evidence

Not applicable: this PR changes command-line help and Markdown usage text, not a rendered frontend or extension UI.

Notes

• The handler behavior is intentionally unchanged: --linked-issue still takes precedence over --issue.