feat(mcp): local stdio MCP server — scan_repo tool (thin proof)

Reuses the extension's pipeline (fetcher/prompt/parser) verbatim; only the
env-key Anthropic call is MCP-specific. Local stdio, BYO-key, no backend.

Author: New1Direction

mcp/README.md

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+# RepoLens MCP server
+
+A local [MCP](https://modelcontextprotocol.io) server that exposes RepoLens's repo
+analysis as a tool. An LLM client (Claude Desktop, Cursor, etc.) calls `scan_repo`
+and gets RepoLens's verdict-first JSON back — ready to render as components.
+
+This is a **thin proof**: one tool, GitHub-only, Anthropic-only. It reuses the
+extension's own pipeline (`fetcher.js` → `prompt.js` → `parser.js`); only the
+provider call is MCP-specific.
+
+## What stays true
+
+- **Local only.** Runs over stdio, spawned by your client. No hosted backend.
+- **Bring your own key.** The Anthropic key comes from the environment, never from
+  a server or the extension's storage. Nothing phones home.
+
+## Tool
+
+### `scan_repo({ repo })`
+
+`repo` is `owner/name` or a GitHub URL. Returns the analysis JSON:
+
+```json
+{
+  "repoId": "honojs/hono",
+  "platform": "github",
+  "language": "TypeScript",
+  "license": "MIT",
+  "stars": 21000,
+  "description": "Small, fast web framework for the edges.",
+  "fit": "strong",
+  "health": { "score": 92 },
+  "pros": ["..."],
+  "cons": ["..."],
+  "red_flags": [],
+  "capabilities": ["routing", "middleware", "edge"]
+}
+```
+
+## Setup
+
+```bash
+cd mcp
+npm install
+export ANTHROPIC_API_KEY=sk-ant-...     # required
+export ANTHROPIC_MODEL=claude-sonnet-4-6 # optional override
+node server.js                           # speaks MCP over stdio
+```
+
+### Add to Claude Desktop
+
+In `claude_desktop_config.json`:
+
+```json
+{
+  "mcpServers": {
+    "repolens": {
+      "command": "node",
+      "args": ["/absolute/path/to/repolens/mcp/server.js"],
+      "env": { "ANTHROPIC_API_KEY": "sk-ant-..." }
+    }
+  }
+}
+```
+
+## Notes
+
+- Unauthenticated GitHub requests are rate-limited. For heavier use, a `GITHUB_TOKEN`
+  pass-through is a follow-up.
+- Next tools (follow-ups): `blueprint_scene` (the nodes/edges graph), `deep_dive`,
+  multi-provider, and npm / PyPI / GitLab.

mcp/anthropic.js

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+// Minimal Anthropic Messages API call for the MCP server. The key comes from the
+// environment (ANTHROPIC_API_KEY) — never chrome.storage, never a hosted backend —
+// so the server stays local and bring-your-own-key. Mirrors the extension's
+// callAnthropic shape (background.js): same endpoint, version header, and default model.
+
+const ENDPOINT = 'https://api.anthropic.com/v1/messages';
+const DEFAULT_MODEL = 'claude-sonnet-4-6';
+
+/**
+ * @param {string} prompt - the fully-assembled analysis prompt.
+ * @returns {Promise<string>} the model's text response.
+ */
+export async function callAnthropic(prompt) {
+  const key = process.env.ANTHROPIC_API_KEY;
+  if (!key) throw new Error('ANTHROPIC_API_KEY is not set in the environment');
+  const model = process.env.ANTHROPIC_MODEL || DEFAULT_MODEL;
+
+  const res = await fetch(ENDPOINT, {
+    method: 'POST',
+    headers: {
+      'content-type': 'application/json',
+      'anthropic-version': '2023-06-01',
+      'x-api-key': key,
+    },
+    body: JSON.stringify({
+      model,
+      max_tokens: 4096,
+      messages: [{ role: 'user', content: prompt }],
+    }),
+  });
+
+  if (!res.ok) {
+    const detail = await res.text().catch(() => '');
+    throw new Error(`Anthropic API ${res.status}: ${detail.slice(0, 300)}`);
+  }
+
+  const data = await res.json();
+  const text = (data.content || []).map((b) => b.text || '').join('').trim();
+  if (!text) throw new Error('Anthropic returned an empty response');
+  return text;
+}