feat: skip memory startup when Codex rate limits are low (#19990)

## Why

Memory startup runs in the background after an eligible turn, but it can
consume Codex backend quota at exactly the wrong time: when the user is
already near a rate-limit boundary. This PR adds a guard so the memory
pipeline backs off when the Codex rate-limit snapshot says the remaining
budget is too low.

## What Changed

- Added `memories.min_rate_limit_remaining_percent` with a default of
`25`, clamped to `0..=100`, and regenerated `core/config.schema.json`.
- Added `codex-rs/memories/write/src/guard.rs`, which fetches Codex
backend rate limits before memory startup and skips phase 1 / phase 2
when the Codex limit is reached or either tracked window is above the
configured usage ceiling.
- Keeps startup best-effort: non-Codex auth or rate-limit fetch/client
failures preserve the existing memory startup behavior.
- Records a `codex.memory.startup` counter with
`status=skipped_rate_limit` when startup is skipped.
- Added config parsing/clamping coverage and guard unit tests.

## Verification

- Added `codex-rs/memories/write/src/guard_tests.rs` for threshold,
primary/secondary window, and reached-limit behavior.
- Added config tests for TOML parsing and clamping.

codex-rs/config/src/types.rs

@@ -32,6 +32,7 @@ pub const DEFAULT_OTEL_ENVIRONMENT: &str = "dev";
 pub const DEFAULT_MEMORIES_MAX_ROLLOUTS_PER_STARTUP: usize = 2;
 pub const DEFAULT_MEMORIES_MAX_ROLLOUT_AGE_DAYS: i64 = 10;
 pub const DEFAULT_MEMORIES_MIN_ROLLOUT_IDLE_HOURS: i64 = 6;
+pub const DEFAULT_MEMORIES_MIN_RATE_LIMIT_REMAINING_PERCENT: i64 = 25;
 pub const DEFAULT_MEMORIES_MAX_RAW_MEMORIES_FOR_CONSOLIDATION: usize = 256;
 pub const DEFAULT_MEMORIES_MAX_UNUSED_DAYS: i64 = 30;
 const MIN_MEMORIES_MAX_RAW_MEMORIES_FOR_CONSOLIDATION: usize = 1;
@@ -204,6 +205,9 @@ pub struct MemoriesToml {
     pub max_rollouts_per_startup: Option<usize>,
     /// Minimum idle time between last thread activity and memory creation (hours). > 12h recommended.
     pub min_rollout_idle_hours: Option<i64>,
+    /// Minimum remaining percentage required in Codex rate-limit windows before memory startup runs.
+    #[schemars(range(min = 0, max = 100))]
+    pub min_rate_limit_remaining_percent: Option<i64>,
     /// Model used for thread summarisation.
     pub extract_model: Option<String>,
     /// Model used for memory consolidation.
@@ -221,6 +225,7 @@ pub struct MemoriesConfig {
     pub max_rollout_age_days: i64,
     pub max_rollouts_per_startup: usize,
     pub min_rollout_idle_hours: i64,
+    pub min_rate_limit_remaining_percent: i64,
     pub extract_model: Option<String>,
     pub consolidation_model: Option<String>,
 }
@@ -236,6 +241,7 @@ impl Default for MemoriesConfig {
             max_rollout_age_days: DEFAULT_MEMORIES_MAX_ROLLOUT_AGE_DAYS,
             max_rollouts_per_startup: DEFAULT_MEMORIES_MAX_ROLLOUTS_PER_STARTUP,
             min_rollout_idle_hours: DEFAULT_MEMORIES_MIN_ROLLOUT_IDLE_HOURS,
+            min_rate_limit_remaining_percent: DEFAULT_MEMORIES_MIN_RATE_LIMIT_REMAINING_PERCENT,
             extract_model: None,
             consolidation_model: None,
         }
@@ -277,6 +283,10 @@ impl From<MemoriesToml> for MemoriesConfig {
                 .min_rollout_idle_hours
                 .unwrap_or(defaults.min_rollout_idle_hours)
                 .clamp(1, 48),
+            min_rate_limit_remaining_percent: toml
+                .min_rate_limit_remaining_percent
+                .unwrap_or(defaults.min_rate_limit_remaining_percent)
+                .clamp(0, 100),
             extract_model: toml.extract_model,
             consolidation_model: toml.consolidation_model,
         }

codex-rs/config/src/types_tests.rs

@@ -59,3 +59,30 @@ fn memories_config_clamps_count_limits_to_nonzero_values() {
         }
     );
 }
+
+#[test]
+fn memories_config_clamps_rate_limit_remaining_threshold() {
+    let config = MemoriesConfig::from(MemoriesToml {
+        min_rate_limit_remaining_percent: Some(101),
+        ..Default::default()
+    });
+    assert_eq!(
+        config,
+        MemoriesConfig {
+            min_rate_limit_remaining_percent: 100,
+            ..MemoriesConfig::default()
+        }
+    );
+
+    let config = MemoriesConfig::from(MemoriesToml {
+        min_rate_limit_remaining_percent: Some(-1),
+        ..Default::default()
+    });
+    assert_eq!(
+        config,
+        MemoriesConfig {
+            min_rate_limit_remaining_percent: 0,
+            ..MemoriesConfig::default()
+        }
+    );
+}