[codex] Remove legacy ListSkills op

[pakrym-oai]

Why

skills/list is already exposed through app-server v2 and covered by the app-server test suite. Keeping the separate core Op::ListSkills path leaves a duplicate legacy protocol surface that no longer needs to be maintained.

What Changed

• Removed Op::ListSkills and EventMsg::ListSkillsResponse from the core protocol.
• Deleted the corresponding core session handler and stale core integration tests.
• Removed rollout/MCP ignore branches and protocol v1 docs references for the deleted event/op.
• Left app-server skills/list and its existing coverage intact.

Validation

• cargo test -p codex-protocol
• cargo test -p codex-core --test all suite::skills
• cargo check -p codex-mcp-server -p codex-rollout -p codex-rollout-trace
• just fix -p codex-core

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 01e6ce7f02

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".