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>

internal/api/handlers_exec.go

@@ -119,6 +119,8 @@ func (h *execHandler) Exec(w http.ResponseWriter, r *http.Request) {
 		Cmd:        req.Cmd,
 		Args:       req.Args,
 		TimeoutSec: req.TimeoutSec,
+		Envs:       req.Envs,
+		Cwd:        req.Cwd,
 	}))
 	if err != nil {
 		status, code, msg := agentErrToHTTP(err)