fix: resolve bugs and DRY violations in sandbox manager and API handlers

- Fix createFromSnapshot discarding memoryMB param (balloon optimization was dead)
- Fix double dm-snapshot removal in Pause() cleanupPauseFailure path
- Fix DestroySandbox RPC mapping all errors to CodeNotFound
- Fix handleFailed event consumer missing pausing/resuming → error transitions
- Fix stream resource leak in StreamUpload on early-return paths
- Add envs/cwd fields to ExecRequest proto for foreground exec parity
- Extract createResources rollback helper to eliminate 4x duplicated teardown
- Remove unused chClient.ping method
- Add .mcp.json to gitignore

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

proto/hostagent/hostagent.proto

@@ -222,6 +222,10 @@ message ExecRequest {
   repeated string args = 3;
   // Timeout for the command in seconds (default: 30).
   int32 timeout_sec = 4;
+  // Environment variables to set for the command.
+  map<string, string> envs = 5;
+  // Working directory for the command.
+  string cwd = 6;
 }
 
 message ExecResponse {