[codex] Add workspace owner usage nudge UI (#18221)

## Summary

Third PR in the split from #17956. Stacked on #18220.

- shows workspace-owner/member-specific rate-limit messages behind
`workspace_owner_usage_nudge`
- prompts workspace members to notify the owner or request a usage-limit
increase
- sends the confirmed nudge through the app-server API and renders
completion feedback
- adds focused TUI snapshot coverage for prompts and completion states
- feature gate

## Validation

- `cargo test -p codex-backend-client`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server rate_limits`
- `cargo test -p codex-tui workspace_`
- `cargo test -p codex-tui status_`
- `just fmt`
- `just fix -p codex-backend-client`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`
- `just fix -p codex-tui`

codex-rs/core/config.schema.json

@@ -538,6 +538,9 @@
             },
             "workspace_dependencies": {
               "type": "boolean"
+            },
+            "workspace_owner_usage_nudge": {
+              "type": "boolean"
             }
           },
           "type": "object"
@@ -2510,6 +2513,9 @@
         },
         "workspace_dependencies": {
           "type": "boolean"
+        },
+        "workspace_owner_usage_nudge": {
+          "type": "boolean"
         }
       },
       "type": "object"