[codex] Remove legacy ListSkills op (#21282)

## 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`

codex-rs/mcp-server/src/codex_tool_runner.rs

@@ -335,7 +335,6 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::McpToolCallBegin(_)
                     | EventMsg::McpToolCallEnd(_)
                     | EventMsg::McpListToolsResponse(_)
-                    | EventMsg::ListSkillsResponse(_)
                     | EventMsg::RealtimeConversationListVoicesResponse(_)
                     | EventMsg::ExecCommandBegin(_)
                     | EventMsg::TerminalInteraction(_)