[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/core/src/config/config_tests.rs

@@ -460,6 +460,90 @@ direct_only_tool_namespaces = ["mcp__history", "mcp__notes"]
     Ok(())
 }
 
+#[tokio::test]
+async fn load_config_resolves_token_budget_config() -> std::io::Result<()> {
+    for (config_toml, expected) in [
+        (
+            "[features]\ntoken_budget = true\n",
+            TokenBudgetConfig::default(),
+        ),
+        (
+            r#"
+[features.token_budget]
+enabled = true
+reminder_threshold_tokens = 16000
+reminder_message_template = "Custom reminder: {n_remaining} tokens."
+"#,
+            TokenBudgetConfig {
+                reminder_threshold_tokens: Some(16_000),
+                reminder_message_template: "Custom reminder: {n_remaining} tokens.".to_string(),
+            },
+        ),
+    ] {
+        let codex_home = tempdir()?;
+        let config_toml = toml::from_str(config_toml).expect("TOML should deserialize");
+        let config = Config::load_from_base_config_with_overrides(
+            config_toml,
+            ConfigOverrides::default(),
+            codex_home.abs(),
+        )
+        .await?;
+
+        assert!(config.features.enabled(Feature::TokenBudget));
+        assert_eq!(config.token_budget, Some(expected));
+    }
+    Ok(())
+}
+
+#[tokio::test]
+async fn load_config_rejects_invalid_token_budget_reminder_template() -> std::io::Result<()> {
+    for reminder_message_template in [
+        String::new(),
+        "x".repeat(TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE_MAX_BYTES + 1),
+    ] {
+        let codex_home = tempdir()?;
+        let config_toml = toml::from_str(&format!(
+            "[features.token_budget]\nenabled = true\nreminder_message_template = {reminder_message_template:?}\n"
+        ))
+        .expect("TOML should deserialize");
+        let error = Config::load_from_base_config_with_overrides(
+            config_toml,
+            ConfigOverrides::default(),
+            codex_home.abs(),
+        )
+        .await
+        .expect_err("invalid reminder template should be rejected");
+
+        assert_eq!(error.kind(), std::io::ErrorKind::InvalidInput);
+    }
+    Ok(())
+}
+
+#[tokio::test]
+async fn load_config_rejects_non_positive_token_budget_reminder_threshold() -> std::io::Result<()> {
+    for reminder_threshold_tokens in [-1, 0] {
+        let codex_home = tempdir()?;
+        let config_toml = toml::from_str(&format!(
+            "[features.token_budget]\nenabled = true\nreminder_threshold_tokens = {reminder_threshold_tokens}\n"
+        ))
+        .expect("TOML should deserialize");
+        let error = Config::load_from_base_config_with_overrides(
+            config_toml,
+            ConfigOverrides::default(),
+            codex_home.abs(),
+        )
+        .await
+        .expect_err("non-positive reminder threshold should be rejected");
+
+        assert_eq!(error.kind(), std::io::ErrorKind::InvalidInput);
+        assert_eq!(
+            error.to_string(),
+            "features.token_budget.reminder_threshold_tokens must be positive"
+        );
+    }
+    Ok(())
+}
+
 #[tokio::test]
 async fn load_config_resolves_rollout_budget() -> std::io::Result<()> {
     let codex_home = tempdir()?;