feat(TRUEREF-0011): implement MCP server with stdio transport

Adds a Model Context Protocol server that exposes resolve-library-id
and query-docs tools via stdio, with tool schemas identical to context7
for drop-in compatibility with Claude Code, Cursor, and Zed.

- src/mcp/index.ts — server entry point (io.github.trueref/trueref)
- src/mcp/client.ts — HTTP client for TrueRef REST API (TRUEREF_API_URL)
- src/mcp/tools/resolve-library-id.ts — library search tool handler
- src/mcp/tools/query-docs.ts — documentation retrieval tool handler
- src/mcp/index.test.ts — integration tests spawning real server subprocess
- .claude/rules/trueref.md — Claude Code rule file for MCP usage
- package.json: mcp:start script using tsx

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/rules/trueref.md

@@ -0,0 +1,11 @@
+---
+description: Use TrueRef to retrieve documentation for indexed libraries
+alwaysApply: true
+---
+
+When answering questions about indexed libraries, always use the TrueRef MCP tools:
+1. Call `resolve-library-id` with the library name and the user's question to get the library ID
+2. Call `query-docs` with the library ID and question to retrieve relevant documentation
+3. Use the returned documentation to answer the question accurately
+
+Never rely on training data alone for library APIs that may have changed.