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.v2.schemas.json

@@ -4110,12 +4110,17 @@
         "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"
     },