Require content kinds for extension prompt fragments (#40177)

## What changed

- Require each `PromptFragment` to include a producer-owned `ContentItemKind`.
- Re-export `ContentItemKind` from `codex_extension_api` and assign stable kinds to skills, memories, examples, and test contributors.
- Limit prompt slots to developer policy and developer capabilities, and combine contributed fragments into a single developer update.

## Testing

- Update extension registry tests to verify fragment ordering with the new content-kind metadata.

GitOrigin-RevId: ef8624a9b4ae9dce025bca9c0954d91dd8075e85

codex-rs/ext/skills/src/extension.rs

@@ -9,6 +9,7 @@ use codex_exec_server::FileSystemSandboxContext;
 use codex_exec_server::LOCAL_ENVIRONMENT_ID;
 use codex_exec_server::ResolvedSelectedCapabilityRoot;
 use codex_extension_api::ConfigContributor;
+use codex_extension_api::ContentItemKind;
 use codex_extension_api::ContextContributor;
 use codex_extension_api::ContextualUserFragment;
 use codex_extension_api::ExtensionData;
@@ -236,7 +237,12 @@ where
             }
             rendered
                 .fragment
-                .map(|fragment| PromptFragment::developer_capability(fragment.render()))
+                .map(|fragment| {
+                    PromptFragment::developer_capability(
+                        fragment.render(),
+                        ContentItemKind("skills.catalog".to_string()),
+                    )
+                })
                 .into_iter()
                 .collect()
         })