Port envd from e2b with internalized shared packages and Connect RPC

- Copy envd source from e2b-dev/infra, internalize shared dependencies
  into envd/internal/shared/ (keys, filesystem, id, smap, utils)
- Switch from gRPC to Connect RPC for all envd services
- Update module paths to git.omukk.dev/wrenn/{sandbox,sandbox/envd}
- Add proto specs (process, filesystem) with buf-based code generation
- Implement full envd: process exec, filesystem ops, port forwarding,
  cgroup management, MMDS integration, and HTTP API
- Update main module dependencies (firecracker SDK, pgx, goose, etc.)
- Remove placeholder .gitkeep files replaced by real implementations

envd/internal/permissions/keepalive.go

@@ -0,0 +1,29 @@
+package permissions
+
+import (
+	"strconv"
+	"time"
+
+	"connectrpc.com/connect"
+)
+
+const defaultKeepAliveInterval = 90 * time.Second
+
+func GetKeepAliveTicker[T any](req *connect.Request[T]) (*time.Ticker, func()) {
+	keepAliveIntervalHeader := req.Header().Get("Keepalive-Ping-Interval")
+
+	var interval time.Duration
+
+	keepAliveIntervalInt, err := strconv.Atoi(keepAliveIntervalHeader)
+	if err != nil {
+		interval = defaultKeepAliveInterval
+	} else {
+		interval = time.Duration(keepAliveIntervalInt) * time.Second
+	}
+
+	ticker := time.NewTicker(interval)
+
+	return ticker, func() {
+		ticker.Reset(interval)
+	}
+}