feat: Budget skill metadata and surface trimming as a warning (#18298)

Cap the model-visible skills section to a small share of the context
window, with a fallback character budget, and keep only as many implicit
skills as fit within that budget.

Emit a non-fatal warning when enabled skills are omitted, and add a new
app-server warning notification

Record thread-start skill metrics for total enabled skills, kept skills,
and whether truncation happened

---------

Co-authored-by: Matthew Zeng <mzeng@openai.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/skills.rs

@@ -21,11 +21,13 @@ pub use codex_core_skills::SkillError;
 pub use codex_core_skills::SkillLoadOutcome;
 pub use codex_core_skills::SkillMetadata;
 pub use codex_core_skills::SkillPolicy;
+pub use codex_core_skills::SkillRenderReport;
 pub use codex_core_skills::SkillsLoadInput;
 pub use codex_core_skills::SkillsManager;
 pub use codex_core_skills::build_skill_name_counts;
 pub use codex_core_skills::collect_env_var_dependencies;
 pub use codex_core_skills::config_rules;
+pub use codex_core_skills::default_skill_metadata_budget;
 pub use codex_core_skills::detect_implicit_skill_invocation_for_command;
 pub use codex_core_skills::filter_skill_load_outcome_for_product;
 pub use codex_core_skills::injection;
@@ -37,6 +39,7 @@ pub use codex_core_skills::manager;
 pub use codex_core_skills::model;
 pub use codex_core_skills::remote;
 pub use codex_core_skills::render;
+pub use codex_core_skills::render::SkillRenderSideEffects;
 pub use codex_core_skills::render_skills_section;
 pub use codex_core_skills::system;