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/app-server/src/codex_message_processor.rs

@@ -4314,6 +4314,7 @@ impl CodexMessageProcessor {
                 thread_id,
                 thread: codex_thread,
                 session_configured,
+                ..
             }) => {
                 let SessionConfiguredEvent { rollout_path, .. } = session_configured;
                 let Some(rollout_path) = rollout_path else {