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/scripts/sync-engine.sh

@@ -11,15 +11,18 @@ SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 MCP_DIR="$(cd "${SCRIPT_DIR}/.." && pwd)"
 REPO_ROOT="$(cd "${MCP_DIR}/.." && pwd)"
 ENGINE_SRC="${REPO_ROOT}/skills/last30days/scripts"
-VENDORED="${MCP_DIR}/vendored"
+# Embed path must live inside the consuming package (Go //go:embed cannot
+# reach outside its own directory tree), so vendored/ sits under engine/.
+VENDORED="${MCP_DIR}/internal/engine/vendored"
 
 if [ ! -f "${ENGINE_SRC}/last30days.py" ]; then
   echo "sync-engine: ${ENGINE_SRC}/last30days.py not found" >&2
   exit 1
 fi
 
-rm -rf "${VENDORED}"
 mkdir -p "${VENDORED}"
+# Clear stale content while keeping the .gitkeep that anchors the embed path.
+find "${VENDORED}" -mindepth 1 -not -name ".gitkeep" -delete
 
 # Copy the entry script and the lib/ tree (modules + lib/vendor/).
 cp "${ENGINE_SRC}/last30days.py" "${VENDORED}/last30days.py"