Clarify model-generated and legacy app path types (#28577)

## Why

`ApiPathString` kind of implies that it can be used anywhere we pull a
path out of JSON, but it's not really appropriate for tool arguments
when the model might generate relative paths.

Prefer `String` for model-generated paths and we can handle the
conversion per feature for now and define a shared abstraction later if
it makes sense.

# What

Rename `ApiPathString` to `AppLegacyPathString` to clarify its role.

Expand the `path-types` skill to tell the model to leave tool args as
bare strings.

codex-rs/app-server-protocol/schema/json/ServerNotification.json

@@ -107,7 +107,7 @@
         "read": {
           "description": "This will be removed in favor of `entries`.",
           "items": {
-            "$ref": "#/definitions/ApiPathString"
+            "$ref": "#/definitions/LegacyAppPathString"
           },
           "type": [
             "array",
@@ -117,7 +117,7 @@
         "write": {
           "description": "This will be removed in favor of `entries`.",
           "items": {
-            "$ref": "#/definitions/ApiPathString"
+            "$ref": "#/definitions/LegacyAppPathString"
           },
           "type": [
             "array",
@@ -164,9 +164,6 @@
     "AgentPath": {
       "type": "string"
     },
-    "ApiPathString": {
-      "type": "string"
-    },
     "AppBranding": {
       "description": "EXPERIMENTAL - app metadata returned by app-list APIs.",
       "properties": {
@@ -1345,7 +1342,7 @@
         {
           "properties": {
             "path": {
-              "$ref": "#/definitions/ApiPathString"
+              "$ref": "#/definitions/LegacyAppPathString"
             },
             "type": {
               "enum": [
@@ -2339,6 +2336,9 @@
       ],
       "type": "object"
     },
+    "LegacyAppPathString": {
+      "type": "string"
+    },
     "McpServerOauthLoginCompletedNotification": {
       "properties": {
         "error": {