feat: MCP read server — agents query KB via MCP protocol

[rajnavakoti]

Description

Add an MCP (Model Context Protocol) server that exposes the KB as read-only tools for AI agents. Any MCP-compatible client (Claude Code, Cursor, Windsurf) can connect and query entities, ask questions, get gap reports, and browse the knowledge graph. The FastAPI endpoints (/ask, /stats, /health) already exist — this wraps them in MCP protocol.

Acceptance Criteria

• MCP server starts via cb mcp-serve or as part of cb serve --mcp
• Tool: query_kb(question) — ask a natural language question, get grounded answer with citations
• Tool: get_entity(type, id) — retrieve a specific entity by type and ID
• Tool: list_entities(type?) — list entities, optionally filtered by type
• Tool: get_gaps() — return known gaps (MISSING + INCOMPLETE items from last eval)
• Tool: get_stats() — return KB statistics (entity counts by type, coverage scores)
• MCP server config can be added to .mcp.json or ~/.claude.json for auto-discovery
• Works with the block scoping system (serves one block at a time or accepts block parameter)

Out of Scope

• MCP write tools (separate P1 issue)
• Authentication/authorization
• Multi-block simultaneous serving