Harden exec-server unified-exec follow-up

- fall back to local when sandboxed exec cannot be modeled remotely
- use server-issued process ids for remote session continuations
- retain symlink fidelity across fs/readDirectory plumbing
- clean up exited exec-server processes after retention

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -7466,12 +7466,17 @@
           "isFile": {
             "description": "Whether this entry resolves to a regular file.",
             "type": "boolean"
+          },
+          "isSymlink": {
+            "description": "Whether this entry is a symlink.",
+            "type": "boolean"
           }
         },
         "required": [
           "fileName",
           "isDirectory",
-          "isFile"
+          "isFile",
+          "isSymlink"
         ],
         "type": "object"
       },