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/config/src/skills_config.rs

@@ -1,6 +1,7 @@
 //! Skill-related configuration types shared across crates.
 
 use std::collections::HashSet;
+use std::num::NonZeroUsize;
 
 use crate::ConfigLayerSource;
 use crate::ConfigLayerStack;
@@ -36,6 +37,11 @@ pub struct SkillsConfig {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub include_instructions: Option<bool>,
 
+    /// Maximum tokens used by the available-skills catalog. Defaults to 2% of
+    /// the model context window and is capped at 10,000 tokens when set.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub max_context_tokens: Option<NonZeroUsize>,
+
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub config: Vec<SkillConfig>,
 }