Remove the codex-core-skills crate (#37505)

## What changed

- Move `SkillLoadOutcome` and its implicit-path indexing into
  `codex-skills-extension`.
- Import shared skill types directly from `codex-skills` and keep the skill
  prompt size limit with the extension renderer that uses it.
- Remove `codex-core-skills` from the Cargo workspace and dependent crates.

GitOrigin-RevId: 4e9e84909fa2f692bcc94af990bb4671affd776f

codex-rs/ext/skills/src/render.rs

@@ -1,6 +1,5 @@
 use std::borrow::Cow;
 
-use codex_core_skills::MAX_SKILL_PROMPT_BYTES;
 use codex_protocol::protocol::SkillScope;
 use codex_utils_string::approx_token_count;
 use codex_utils_string::take_bytes_at_char_boundary;
@@ -15,6 +14,7 @@ use crate::fragments::AvailableSkillsInstructions;
 use crate::host_aliases::shared_host_alias_roots;
 
 const DEFAULT_SKILL_METADATA_CHAR_BUDGET: usize = 8_000;
+const MAX_SKILL_PROMPT_BYTES: usize = 8_000;
 const SKILL_METADATA_CONTEXT_WINDOW_PERCENT: usize = 2;
 const MAX_CATALOG_SKILL_DESCRIPTION_CHARS: usize = 1_024;
 const TRUNCATED_SKILL_DESCRIPTION_SUFFIX: &str = "...";