[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/context/mod.rs

@@ -73,6 +73,7 @@ pub(crate) use rollout_budget::RolloutBudgetContext;
 pub(crate) use subagent_notification::SubagentNotification;
 pub(crate) use token_budget_context::TokenBudgetContext;
 pub(crate) use token_budget_context::TokenBudgetRemainingContext;
+pub(crate) use token_budget_context::TokenBudgetReminder;
 pub(crate) use turn_aborted::TurnAborted;
 pub(crate) use user_instructions::UserInstructions;
 pub(crate) use user_shell_command::UserShellCommand;

codex-rs/core/src/context/token_budget_context.rs

@@ -99,3 +99,34 @@ impl ContextualUserFragment for TokenBudgetRemainingContext {
         }
     }
 }
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct TokenBudgetReminder {
+    message: String,
+}
+
+impl TokenBudgetReminder {
+    pub(crate) fn new(message_template: &str, n_remaining: i64) -> Self {
+        Self {
+            message: message_template.replace("{n_remaining}", &n_remaining.to_string()),
+        }
+    }
+}
+
+impl ContextualUserFragment for TokenBudgetReminder {
+    fn role(&self) -> &'static str {
+        "developer"
+    }
+
+    fn markers(&self) -> (&'static str, &'static str) {
+        Self::type_markers()
+    }
+
+    fn type_markers() -> (&'static str, &'static str) {
+        ("<token_budget>\n", "\n</token_budget>")
+    }
+
+    fn body(&self) -> String {
+        self.message.clone()
+    }
+}