Gate goal tools by thread eligibility (#24925)

## Why

Goal tools create and update goal state for a persistent thread. The
extension was only checking whether goals were enabled before
advertising those tools, which meant they could be surfaced in contexts
that should not receive thread goal controls: ephemeral threads without
persistent thread state and review subagents.

Those sessions can still run the goal extension lifecycle, but the
thread tools should only be visible when the current thread can safely
use them.

## What changed

- Adds a `GoalRuntimeConfig` that separates goal enablement from whether
goal tools are available for the current thread.
- Computes tool eligibility on thread start from
`persistent_thread_state_available` and `SessionSource`, hiding tools
for review subagents.
- Uses `GoalRuntimeHandle::tools_visible()` when contributing thread
tools so enabled runtime state does not automatically imply tool
exposure.
- Adds backend coverage for hiding goal tools on ephemeral threads and
review subagents.

## Testing

- Added `goal_tools_hidden_for_ephemeral_threads`.
- Added `goal_tools_hidden_for_review_subagents`.

codex-rs/ext/goal/src/extension.rs

@@ -22,6 +22,8 @@ use codex_extension_api::TurnStartInput;
 use codex_extension_api::TurnStopInput;
 use codex_otel::MetricsClient;
 use codex_protocol::ThreadId;
+use codex_protocol::protocol::SessionSource;
+use codex_protocol::protocol::SubAgentSource;
 use codex_protocol::protocol::ThreadGoalStatus;
 use codex_protocol::protocol::TokenUsageInfo;
 
@@ -29,6 +31,7 @@ use crate::accounting::BudgetLimitedGoalDisposition;
 use crate::accounting::GoalAccountingState;
 use crate::events::GoalEventEmitter;
 use crate::metrics::GoalMetrics;
+use crate::runtime::GoalRuntimeConfig;
 use crate::runtime::GoalRuntimeHandle;
 use crate::spec::UPDATE_GOAL_TOOL_NAME;
 use crate::steering::budget_limit_steering_item;
@@ -85,6 +88,11 @@ where
 {
     async fn on_thread_start(&self, input: ThreadStartInput<'_, C>) {
         let enabled = (self.goals_enabled)(input.config);
+        let tools_available_for_thread = input.persistent_thread_state_available
+            && !matches!(
+                input.session_source,
+                SessionSource::SubAgent(SubAgentSource::Review)
+            );
         input
             .thread_store
             .insert(GoalExtensionConfig::from_enabled(enabled));
@@ -102,7 +110,10 @@ where
                 self.metrics.clone(),
                 self.thread_manager.clone(),
                 accounting_state,
-                enabled,
+                GoalRuntimeConfig {
+                    enabled,
+                    tools_available_for_thread,
+                },
             )
         });
         runtime.set_enabled(enabled);
@@ -330,7 +341,7 @@ where
         let Some(runtime) = goal_runtime_handle(thread_store) else {
             return Vec::new();
         };
-        if !runtime.is_enabled() {
+        if !runtime.tools_visible() {
             return Vec::new();
         }