[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/docs/protocol_v1.md

@@ -70,7 +70,6 @@ For complete documentation of the `Op` and `EventMsg` variants, refer to [protoc
   - `Op::Interrupt` – Interrupts a running turn
   - `Op::ExecApproval` – Approve or deny code execution
   - `Op::UserInputAnswer` – Provide answers for a `request_user_input` tool call
-  - `Op::ListSkills` – Request skills for one or more cwd values (optionally `force_reload`)
   - `Op::UserTurn` and `Op::OverrideTurnContext` accept an optional `personality` override that updates the model’s communication style
 
 Valid `personality` values are `friendly`, `pragmatic`, and `none`. When `none` is selected, the personality placeholder is replaced with an empty string.
@@ -86,7 +85,6 @@ Valid `personality` values are `friendly`, `pragmatic`, and `none`. When `none`
   - `EventMsg::Error` – A turn stopped with an error
   - `EventMsg::Warning` – A non-fatal warning that the client should surface to the user
   - `EventMsg::TurnComplete` – Contains a `response_id` bookmark for last `response_id` executed by the turn. This can be used to continue the turn at a later point in time, perhaps with additional user input.
-  - `EventMsg::ListSkillsResponse` – Response payload with per-cwd skill entries (`cwd`, `skills`, `errors`)
 
 ### UserInput items