[Phase 2] Document per-stack MCP server recommendations and the opt-in

[github-actions]

Story

As a downstream repo owner,
I want read documented recommended MCP servers per tech stack and how to enable them safely,
so that I can adopt MCP review enrichment for my stack without misconfiguring auth or degradation.

Acceptance Criteria

1. A new doc under docs/pr-review-agent/ documents the mcp_servers input, the MCP_CONFIG format the engine expects, and the Fail-Loud-Never-Fake degradation behavior.
2. It documents a recommended server set per stack: Context7 (real-time library docs; free, stateless, no-auth) for React/Python/Go; GitHub MCP Server (secret scanning GA, dependency scanning public preview) for secret/dependency enrichment; SonarQube MCP for code-quality enrichment -- each with auth/availability caveats grounded in discussion MCP-Powered CI Review Enhancement — Standard #650.
3. It states the security-first sequencing (enable only after the org's action-allowlisting / input-sanitization hardening lands) and that auth-bearing servers source tokens from secrets/env, never from committed config.
4. The new doc is linked from docs/pr-review-agent/documentation-index.md.
5. Docs lint / docs-health-check passes.

Tasks / Subtasks

• Write docs/pr-review-agent/mcp-enrichment.md covering the input, MCP_CONFIG format, degradation behavior, and per-stack recommendations. (AC: test issue from agent #1, Go-live improvements for PR review agent #2, Add @claude delegation, auto-merge, and rebase handling #3)
• Add a link entry in docs/pr-review-agent/documentation-index.md. (AC: Optimize review: small-PR and incremental fast paths #4)
• Run the docs lint / docs-health-check to confirm pass. (AC: feat: add Copilot engine support via REVIEW_ENGINE toggle #5)

Dev Notes

• Place the doc alongside the existing review-agent docs (docs/pr-review-agent/setup.md, implementation.md, pr-review-agent.md) and register it in documentation-index.md, matching how that folder is organized.
• Ground ALL server claims in discussion MCP-Powered CI Review Enhancement — Standard #650 (body + 2026-06-13 weekly update): MCP ecosystem scale, GitHub MCP secret-scanning GA / dependency-scanning preview, Context7 as stateless/no-auth, SonarQube for code-quality. Do not invent servers or capabilities beyond those named.
• Surface the unresolved starter-server choice as an explicit caveat: the design body picks Context7 (lowest friction); the weekly update recommends starting with GitHub MCP secret scanning for security-first alignment. The doc should present both, deferring the pick to the pilot.
• Reflect the actual input shape delivered in story 2 (input name mcp_servers, env MCP_CONFIG, org-var fallback) so the doc and implementation agree.

Project Structure Notes

Docs-only change under docs/pr-review-agent/, the canonical home for review-agent documentation.

References

• docs/pr-review-agent/documentation-index.md
• docs/pr-review-agent/setup.md
• discussion MCP-Powered CI Review Enhancement — Standard #650

Likely target surface

• docs/pr-review-agent/mcp-enrichment.md
• docs/pr-review-agent/documentation-index.md

Story prepared by the BMAD Scrum Master (Bob) for epic #659. Status: ready-for-dev.