feat(mcp): embed Python engine and extract to user cache

U2 of the Claude Desktop .mcpb bundle plan.

- internal/engine/embed.go embeds the vendored Python tree at build time
  via //go:embed all:vendored. The engine package owns the embed because
  Go's directive cannot reach outside its own package directory; sync and
  gitignore paths are updated to match (internal/engine/vendored/ in
  place of mcp/vendored/).
- internal/engine/extract.go materializes the embed into
  <cache>/last30days-pp-mcp/<version>/ with a .version sentinel that
  short-circuits re-extraction. Atomic rename from a .tmp sibling means
  a partial extraction can never be mistaken for complete. Concurrent
  first-call extractions serialize behind a per-cache-dir sync.Once.
- EnsureUserCache honors a LAST30DAYS_CACHE_DIR env override for
  locked-down filesystems; the override is named in extract errors.
- internal/engine/extract_test.go covers happy path, sentinel skip,
  version bump, 10-goroutine race, empty-version rejection, unwritable
  cache parent, and the env override (7 tests, all passing).
- A tracked vendored/.gitkeep anchors the embed path so the directive
  matches even before scripts/sync-engine.sh runs.

mcp/internal/engine/embed.go

@@ -0,0 +1,26 @@
+// Package engine wraps the vendored Python last30days engine. The engine
+// is embedded at build time via //go:embed and extracted into a per-user
+// cache directory on first use, then invoked through python3 in a
+// subprocess. Consumers should call EnsureUserCache to materialize the
+// engine and Run to execute it.
+package engine
+
+import (
+	"embed"
+	"io/fs"
+)
+
+// EngineSourceDir is the embed root inside the binary. scripts/sync-engine.sh
+// mirrors skills/last30days/scripts/ into this directory before each build.
+// The all: prefix preserves files starting with "." or "_" so the .gitkeep
+// anchor file survives - without it the embed would error before sync runs.
+//
+//go:embed all:vendored
+var vendored embed.FS
+
+// EngineFS returns the embedded engine as a filesystem rooted at the
+// vendored/ directory contents (so callers see "last30days.py" at the
+// root, not "vendored/last30days.py").
+func EngineFS() (fs.FS, error) {
+	return fs.Sub(vendored, "vendored")
+}