[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/features/src/lib.rs

@@ -193,6 +193,8 @@ pub enum Feature {
     TuiAppServer,
     /// Prevent idle system sleep while a turn is actively running.
     PreventIdleSleep,
+    /// Enable workspace-specific owner nudge copy and prompts in the TUI.
+    WorkspaceOwnerUsageNudge,
     /// Legacy rollout flag for Responses API WebSocket transport experiments.
     ResponsesWebsockets,
     /// Legacy rollout flag for Responses API WebSocket transport v2 experiments.
@@ -968,6 +970,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         },
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::WorkspaceOwnerUsageNudge,
+        key: "workspace_owner_usage_nudge",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::ResponsesWebsockets,
         key: "responses_websockets",