feat(mcp): scaffold Go module under mcp/ for Claude Desktop bundle

Add a new top-level mcp/ Go module that will host the Claude Desktop MCPB
server. U1 of the Claude Desktop .mcpb bundle plan: scaffold only, no
behavior yet.

- mcp/go.mod targets Go 1.22+ with mark3labs/mcp-go as the planned
  dependency (added in U3 when the server wiring lands).
- mcp/scripts/sync-engine.sh mirrors skills/last30days/scripts/ into
  mcp/vendored/ before each build, keeping the Code skill and Desktop
  bundle on the same engine source.
- Root .gitignore excludes mcp/vendored/ and mcp/build/ so the engine
  mirror and cross-compiled binaries stay local.

.gitignore

@@ -26,5 +26,10 @@ htmlcov/
 # build artifact from scripts/build-skill.sh
 /dist/
 
+# Go MCP bundle build outputs - source of truth for vendored/ stays under
+# skills/last30days/scripts/; build/ holds cross-compiled binaries + .mcpb files.
+/mcp/vendored/
+/mcp/build/
+
 # Internal planning docs (ce:plan output) — keep local, don't publish
 docs/plans/