feat(mcp): stdio MCP server with research tool

U3 of the Claude Desktop .mcpb bundle plan.

- internal/engine/run.go invokes python3 with the cached last30days.py,
  forwards os.Environ() so MCPB user_config env-injection reaches the
  engine, sets PYTHONPATH so the lib/ imports resolve, and surfaces
  three distinct error shapes (missing interpreter with install URL,
  non-zero exit with stderr, timeout). RunOptions.PythonPath lets tests
  inject a stub without manipulating PATH.
- internal/engine/run_test.go drives a shell-script stub interpreter
  through happy path, env forwarding, PYTHONPATH, non-zero exit with
  stderr surfacing, timeout, missing python3 (empty PATH), missing
  last30days.py, empty CacheDir, and timeout-env-override parsing.
- internal/tools/research.go registers a single research tool whose
  schema mirrors /last30days <topic> (required topic, optional emit
  enum, optional save bool). Validation failures surface as MCP
  tool errors so Claude sees structured failures instead of transport
  faults; engine extract or run errors fold engine stderr into the
  message so users can diagnose without leaving Desktop.
- internal/tools/research_test.go covers requireString, emitArgument,
  boolArgument, handler-level validation routing, and formatRunError.
- cmd/last30days-pp-mcp/main.go wires NewMCPServer + tools.Register +
  ServeStdio. main.Version is ldflags-stamped at build time and
  namespaces the per-user cache. mcp-go pinned at v0.54.0.

Smoke check: ./build/last30days-pp-mcp answers tools/list with the
research tool plus full schema and read-only/open-world annotations.
go test ./... passes across engine + tools (32 cases).

mcp/cmd/last30days-pp-mcp/main.go

@@ -1,11 +1,39 @@
 // Package main is the entry point for the last30days MCP server bundled
-// as a .mcpb for Claude Desktop. See mcp/README.md for build instructions.
+// as a .mcpb for Claude Desktop. The server registers a single research
+// tool (see internal/tools) and serves it over stdio. See mcp/README.md
+// for build and packaging instructions.
 package main
 
+import (
+	"fmt"
+	"os"
+
+	"github.com/mark3labs/mcp-go/server"
+
+	"github.com/mvanhorn/last30days-skill/mcp/internal/tools"
+)
+
 // Version is stamped at build time via -ldflags "-X main.Version=<tag>".
-// Defaults to "dev" for local builds.
+// It namespaces the per-user cache directory in internal/engine so multiple
+// installed versions can coexist without clobbering each other.
 var Version = "dev"
 
+const (
+	serverName    = "last30days"
+	serverVersion = "1"
+)
+
 func main() {
-	// Wired in U3.
+	s := server.NewMCPServer(
+		serverName,
+		serverVersion,
+		server.WithToolCapabilities(false),
+	)
+
+	tools.Register(s, tools.Config{Version: Version})
+
+	if err := server.ServeStdio(s); err != nil {
+		fmt.Fprintf(os.Stderr, "last30days-pp-mcp: %v\n", err)
+		os.Exit(1)
+	}
 }