[codex] add configurable token budget compaction reminder (#29255)

## Why

The token-budget feature reports coarse remaining-context milestones,
but it does not give the model a configurable wrap-up prompt before
automatic compaction. A strict threshold-crossing check can also miss
resumed or reconfigured windows that are already inside the threshold.

## What changed

- Add structured `[features.token_budget]` configuration for an absolute
`reminder_threshold_tokens` and bounded `reminder_message_template`;
`{n_remaining}` is expanded when the reminder is delivered.
- Compute remaining tokens against the next effective auto-compaction
boundary, including scoped `body_after_prefix` accounting and the full
context-window limit.
- Make reminder delivery level-triggered before and after sampling, with
one-shot state owned by `AutoCompactWindow` and re-armed on compaction,
`new_context`, restore, or history replacement.
- Leave the existing initial full-window token-budget context, 25/50/75%
notices, and token-budget tools unchanged.
- Persist the resolved feature configuration in the session config lock
and regenerate the config schema.

## Validation

- `just test -p codex-core token_budget`
- `just test -p codex-core
token_budget_reminder_emits_after_crossing_compaction_threshold`
- `just test -p codex-core auto_compact_window`
- `just test -p codex-core
lock_contains_prompts_and_materializes_features`
- `just test -p codex-features`
- `just test -p codex-config`

codex-rs/features/src/feature_configs.rs

@@ -73,6 +73,32 @@ impl FeatureConfig for MultiAgentV2ConfigToml {
     }
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq, JsonSchema)]
+#[serde(deny_unknown_fields)]
+pub struct TokenBudgetConfigToml {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub enabled: Option<bool>,
+    /// Number of tokens remaining before auto-compaction when the wrap-up reminder is emitted.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[schemars(range(min = 1))]
+    pub reminder_threshold_tokens: Option<i64>,
+    /// Reminder template. `{n_remaining}` is replaced with the tokens remaining before
+    /// auto-compaction.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[schemars(length(min = 1, max = 1000))]
+    pub reminder_message_template: Option<String>,
+}
+
+impl FeatureConfig for TokenBudgetConfigToml {
+    fn enabled(&self) -> Option<bool> {
+        self.enabled
+    }
+
+    fn set_enabled(&mut self, enabled: bool) {
+        self.enabled = Some(enabled);
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, JsonSchema)]
 #[serde(deny_unknown_fields)]
 pub struct RolloutBudgetConfigToml {

codex-rs/features/src/lib.rs

@@ -26,6 +26,7 @@ pub use feature_configs::NetworkProxyModeToml;
 pub use feature_configs::NetworkProxyUnixSocketPermissionToml;
 use feature_configs::RemovedAppsMcpPathOverrideConfigToml;
 pub use feature_configs::RolloutBudgetConfigToml;
+pub use feature_configs::TokenBudgetConfigToml;
 use legacy::LegacyFeatureToggles;
 pub use legacy::legacy_feature_keys;
 
@@ -632,6 +633,8 @@ pub struct FeaturesToml {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub multi_agent_v2: Option<FeatureToml<MultiAgentV2ConfigToml>>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub token_budget: Option<FeatureToml<TokenBudgetConfigToml>>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub rollout_budget: Option<FeatureToml<RolloutBudgetConfigToml>>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub current_time_reminder: Option<FeatureToml<CurrentTimeReminderConfigToml>>,
@@ -667,6 +670,9 @@ impl FeaturesToml {
         if let Some(enabled) = self.multi_agent_v2.as_ref().and_then(FeatureToml::enabled) {
             entries.insert(Feature::MultiAgentV2.key().to_string(), enabled);
         }
+        if let Some(enabled) = self.token_budget.as_ref().and_then(FeatureToml::enabled) {
+            entries.insert(Feature::TokenBudget.key().to_string(), enabled);
+        }
         if let Some(enabled) = self.rollout_budget.as_ref().and_then(FeatureToml::enabled) {
             entries.insert(Feature::RolloutBudget.key().to_string(), enabled);
         }
@@ -688,6 +694,7 @@ impl FeaturesToml {
         let Self {
             code_mode,
             multi_agent_v2,
+            token_budget,
             rollout_budget,
             current_time_reminder,
             removed_apps_mcp_path_override: _,
@@ -703,6 +710,8 @@ impl FeaturesToml {
                 materialize_resolved_feature_enabled(code_mode, enabled);
             } else if spec.id == Feature::MultiAgentV2 {
                 materialize_resolved_feature_enabled(multi_agent_v2, enabled);
+            } else if spec.id == Feature::TokenBudget {
+                materialize_resolved_feature_enabled(token_budget, enabled);
             } else if spec.id == Feature::RolloutBudget {
                 materialize_resolved_feature_enabled(rollout_budget, enabled);
             } else if spec.id == Feature::CurrentTimeReminder {