feat: Budget skill metadata and surface trimming as a warning (#18298)

Cap the model-visible skills section to a small share of the context
window, with a fallback character budget, and keep only as many implicit
skills as fit within that budget.

Emit a non-fatal warning when enabled skills are omitted, and add a new
app-server warning notification

Record thread-start skill metrics for total enabled skills, kept skills,
and whether truncation happened

---------

Co-authored-by: Matthew Zeng <mzeng@openai.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -4303,6 +4303,26 @@
           "title": "Model/reroutedNotification",
           "type": "object"
         },
+        {
+          "properties": {
+            "method": {
+              "enum": [
+                "warning"
+              ],
+              "title": "WarningNotificationMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/WarningNotification"
+            }
+          },
+          "required": [
+            "method",
+            "params"
+          ],
+          "title": "WarningNotification",
+          "type": "object"
+        },
         {
           "properties": {
             "method": {
@@ -15830,6 +15850,27 @@
         ],
         "type": "string"
       },
+      "WarningNotification": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "message": {
+            "description": "Concise warning message for the user.",
+            "type": "string"
+          },
+          "threadId": {
+            "description": "Optional thread target when the warning applies to a specific thread.",
+            "type": [
+              "string",
+              "null"
+            ]
+          }
+        },
+        "required": [
+          "message"
+        ],
+        "title": "WarningNotification",
+        "type": "object"
+      },
       "WebSearchAction": {
         "oneOf": [
           {