mcpd: Go MCP SDK port — replaces Bun mcp-server tool surface

New cmd/mcpd binary using github.com/modelcontextprotocol/go-sdk
v1.5.0 over stdio transport. Exposes Lakehouse capabilities as MCP
tools: list_datasets, get_manifest, query_sql, embed_text,
search_vectors. Each tool proxies to the gateway via HTTP.

Replaces the MCP-tool subset of the Rust system's Bun mcp-server.ts
(the audit's "split this 2520-line empire" finding from R-005). HTTP
demo routes (the staffing co-pilot UI at /api/intelligence/*,
/headshots/*, etc.) stay Bun until G5 cutover — those are demo-
specific and depend on matrix-indexer signals not yet ported.

Architecture:
  cmd/mcpd/main.go (235 LoC)
    main() reads --gateway flag, builds server via buildServer(),
    runs on StdioTransport. Each tool's args is a typed struct with
    jsonschema tags (the SDK's canonical pattern); reflection
    generates the JSON Schema automatically.

    gatewayClient: thin HTTP wrapper over the configured gateway URL.
    30s per-request timeout. 16 MiB tool-response cap. Non-2xx
    surfaces as IsError CallToolResult (NOT as transport error) so
    the LLM caller sees the error text and can decide how to react.

    proxy() handles GET + POST + JSON body uniformly. errorResult()
    + jsonResult() helpers normalize CallToolResult shape.

  cmd/mcpd/main_test.go (13 test funcs)
    Tests the full MCP wire end-to-end without a subprocess: spin
    up a fake gateway via httptest, build the MCP server pointed at
    it, connect a client via in-memory transports (NewInMemoryTransports),
    call each tool. Each tool gets:
      - happy path (gateway returns 200 → tool returns content)
      - input validation (missing required fields → IsError)
      - upstream error (gateway 4xx → tool returns IsError)
    Plus TestListTools verifies all 5 tools register; TestGatewayUnreachable
    verifies network-level failures surface as IsError, not panics.

Setup for Claude Desktop / Code documented in README:
  {
    "mcpServers": {
      "lakehouse": {
        "command": "/path/to/bin/mcpd",
        "args": ["--gateway", "http://127.0.0.1:3110"]
      }
    }
  }

Verified:
  go test -count=1 ./cmd/mcpd/  — 13/13 green
  just verify                    — vet + test + 9 smokes 35s

Out of scope for this commit:
  - Resources (mcp.AddResource): not needed yet; tools cover the
    interactive surface. Add when an LLM-side use case shows up.
  - Prompts (mcp.AddPrompt): same.
  - Streamable transports (HTTP, SSE): stdio is the universal one;
    streamable can be added with srv.Run(ctx, &mcp.StreamableHTTPHandler{})
    swap if a daemon-mode deploy makes sense.
  - mcpd inside the daemon-supervised stack: it's stdio-only and
    spawned by the MCP client, not run as a service. Adding a
    daemon-mode (HTTP transport on a port) is a follow-up if MCP
    consumers want long-lived sessions.

This is a tool-surface only port. The Bun mcp-server.ts also serves
HTTP demo routes (/api/catalog/datasets, /intelligence/*, /headshots/*)
that depend on the matrix-indexer signals from the Rust system; those
stay Bun until G5 cutover when the staffing co-pilot service ports
to Go.

Direct deps added:
  github.com/modelcontextprotocol/go-sdk v1.5.0

Transitive (resolved by go mod tidy):
  github.com/google/jsonschema-go        v0.4.2
  github.com/yosida95/uritemplate/v3     v3.0.2
  golang.org/x/oauth2                    v0.35.0
  github.com/segmentio/encoding          v0.5.4
  github.com/golang-jwt/jwt/v5           v5.3.1

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.sum

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