Add a configurable skill catalog token budget (#38978)

## What changed

- Add `[skills].max_context_tokens` to override the token budget used to render the available-skills catalog.
- Require a positive value and cap configured budgets at 10,000 tokens.
- Preserve the existing default of 2% of the model context window when the option is unset.

## Testing

- Cover configuration parsing, override behavior, the 10,000-token cap, and catalog rendering within the configured budget.

GitOrigin-RevId: f29ddcb57fc2d800c0006d03e1c33cbe3ab6272d

codex-rs/thread-manager-sample/src/main.rs

@@ -215,6 +215,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         include_apps_instructions: false,
         include_collaboration_mode_instructions: false,
         include_skill_instructions: false,
+        skill_max_context_tokens: None,
         orchestrator_skills_enabled: false,
         orchestrator_mcp_enabled: false,
         include_environment_context: false,