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/Cargo.lock

@@ -2072,6 +2072,7 @@ dependencies = [
  "codex-protocol",
  "codex-skills",
  "codex-utils-absolute-path",
+ "codex-utils-output-truncation",
  "codex-utils-plugins",
  "dirs",
  "dunce",