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/go.mod

@@ -1,3 +1,14 @@
 module github.com/mvanhorn/last30days-skill/mcp
 
-go 1.22
+go 1.25.5
+
+require github.com/mark3labs/mcp-go v0.54.0
+
+require (
+	github.com/google/jsonschema-go v0.4.2 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2 // indirect
+	github.com/spf13/cast v1.7.1 // indirect
+	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
+	golang.org/x/text v0.14.0 // indirect
+)