revert: openai-codex goes back to the compiled-in catalog

Reverts c61977c. The live listing needed no codex CLI on the user's
machine — kigi issued the request itself and `client_version` was a
compiled-in string — but the project keeps the catalog in the binary, so
the fetch path, the `OpenaiCodex` listing dialect, its parser and the
pinned client version are gone again, and the codex `User-Agent` returns
to its previous value.

The models.dev provider rename `kimi-for-coding` → `kimi-code-plan-cn`
rode in the same commit and stays: the regenerated snapshot ships under
the new id.

crates/codegen/kigi-sampling-types/src/types.rs

@@ -1198,19 +1198,11 @@ pub const COPILOT_INITIATOR: &str = "user";
 pub const CODEX_ORIGINATOR: &str = "codex_cli_rs";
 /// `OpenAI-Beta` opt-in the Codex Responses endpoint requires.
 pub const CODEX_OPENAI_BETA: &str = "responses=experimental";
-/// Codex CLI version kigi presents to the Codex backend. The models catalog
-/// (`/codex/models?client_version=`) is GATED on it: an old version answers
-/// with a truncated model list (0.104.0 serves no user-facing model at all),
-/// so this tracks a real upstream release (openai/codex rust-v0.155.1).
-macro_rules! codex_client_version {
-    () => {
-        "0.155.1"
-    };
-}
-pub const CODEX_CLIENT_VERSION: &str = codex_client_version!();
 /// `User-Agent` presented on the Codex path (overrides the default kigi UA,
-/// openai-codex-gated), carrying the same pinned version.
-pub const CODEX_USER_AGENT: &str = concat!("codex_cli_rs/", codex_client_version!());
+/// openai-codex-gated). The Codex backend does not strictly validate the UA
+/// string (Pi ships its own and it works), so this is a stable best-effort
+/// identity, not a pinned build.
+pub const CODEX_USER_AGENT: &str = "codex_cli_rs/0.104.0";
 /// JWT payload claim namespace carrying the ChatGPT account id.
 const CODEX_JWT_AUTH_CLAIM: &str = "https://api.openai.com/auth";
 
@@ -1643,16 +1635,6 @@ mod tests {
     use super::*;
     use serde_json::json;
 
-    /// The catalog gate and the UA must claim the SAME codex version: the
-    /// backend serves a truncated model list to an old client_version.
-    #[test]
-    fn codex_user_agent_carries_the_pinned_client_version() {
-        assert_eq!(
-            CODEX_USER_AGENT,
-            format!("codex_cli_rs/{CODEX_CLIENT_VERSION}")
-        );
-    }
-
     /// The Codex backend rejects `role: system` input outright
     /// (400 `{"detail":"System messages are not allowed"}`) — its system
     /// channel is the top-level `instructions` field, and stateless