fix(codex): restore Version header and single-source the CLI version

The image handler's `version` header was commented out, so Codex image
requests reached chatgpt.com without the Version identity the backend
expects. Restore it and route every Codex identity header through one
constant.

The CLI version now lives on registry codex.transport as `cliVersion`
(the same pattern gemini-cli uses) and is re-exported as CODEX_CLI_VERSION,
so the registry User-Agent, the image handler and the connection test can
no longer drift apart. Bumped 0.136.0 -> 0.154.0 (current stable).

Co-Authored-By: Claude Code <noreply@anthropic.com>

tests/__baseline__/providers-baseline.json

@@ -192,9 +192,10 @@
     "baseUrl": "https://chatgpt.com/backend-api/codex/responses",
     "format": "openai-responses",
     "forceStream": true,
+    "cliVersion": "0.154.0",
     "headers": {
       "originator": "codex_cli_rs",
-      "User-Agent": "codex_cli_rs/0.136.0"
+      "User-Agent": "codex_cli_rs/0.154.0"
     },
     "usage": {
       "url": "https://chatgpt.com/backend-api/wham/usage",

tests/unit/image-generation.test.js

@@ -351,7 +351,7 @@ describe("handleImageGenerationCore", () => {
         headers: expect.objectContaining({
           authorization: "Bearer codex-token",
           "chatgpt-account-id": "account-123",
-          version: "0.136.0",
+          version: "0.154.0",
         }),
       })
     );