Move skill catalog rendering out of core (#36364)

## What changed

- Make the skills extension own the catalog prompt templates and rendering path.
- Remove the duplicate core fallback that injected available skills into initial context.
- Update core integration tests to install the skills extension explicitly when they expect model-visible skill context.

GitOrigin-RevId: bbe6b15c4e776a09c98b7f4166426fe4e58434ba

codex-rs/core-skills/Cargo.toml

@@ -25,7 +25,6 @@ codex-protocol = { workspace = true }
 codex-shell-command = { workspace = true }
 codex-skills = { workspace = true }
 codex-utils-absolute-path = { workspace = true }
-codex-utils-output-truncation = { workspace = true }
 codex-utils-path-uri = { workspace = true }
 codex-utils-plugins = { workspace = true }
 dirs = { workspace = true }