[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/config.schema.json

@@ -669,7 +669,7 @@
               "type": "boolean"
             },
             "token_budget": {
-              "type": "boolean"
+              "$ref": "#/definitions/FeatureToml_for_TokenBudgetConfigToml"
             },
             "tool_call_mcp_elicitation": {
               "type": "boolean"
@@ -967,6 +967,16 @@
         }
       ]
     },
+    "FeatureToml_for_TokenBudgetConfigToml": {
+      "anyOf": [
+        {
+          "type": "boolean"
+        },
+        {
+          "$ref": "#/definitions/TokenBudgetConfigToml"
+        }
+      ]
+    },
     "FeedbackConfigToml": {
       "additionalProperties": false,
       "properties": {
@@ -2836,6 +2846,27 @@
         }
       ]
     },
+    "TokenBudgetConfigToml": {
+      "additionalProperties": false,
+      "properties": {
+        "enabled": {
+          "type": "boolean"
+        },
+        "reminder_message_template": {
+          "description": "Reminder template. `{n_remaining}` is replaced with the tokens remaining before auto-compaction.",
+          "maxLength": 1000,
+          "minLength": 1,
+          "type": "string"
+        },
+        "reminder_threshold_tokens": {
+          "description": "Number of tokens remaining before auto-compaction when the wrap-up reminder is emitted.",
+          "format": "int64",
+          "minimum": 1.0,
+          "type": "integer"
+        }
+      },
+      "type": "object"
+    },
     "ToolSuggestConfig": {
       "additionalProperties": false,
       "properties": {
@@ -4932,7 +4963,7 @@
           "type": "boolean"
         },
         "token_budget": {
-          "type": "boolean"
+          "$ref": "#/definitions/FeatureToml_for_TokenBudgetConfigToml"
         },
         "tool_call_mcp_elicitation": {
           "type": "boolean"