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/core/config.schema.json

@@ -1114,6 +1114,13 @@
           "format": "int64",
           "type": "integer"
         },
+        "min_rate_limit_remaining_percent": {
+          "description": "Minimum remaining percentage required in Codex rate-limit windows before memory startup runs.",
+          "format": "int64",
+          "maximum": 100.0,
+          "minimum": 0.0,
+          "type": "integer"
+        },
         "min_rollout_idle_hours": {
           "description": "Minimum idle time between last thread activity and memory creation (hours). > 12h recommended.",
           "format": "int64",