[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/protocol/src/protocol.rs

@@ -748,19 +748,6 @@ pub enum Op {
     /// enable/disable state) without restarting the thread.
     ReloadUserConfig,
 
-    /// Request the list of skills for the provided `cwd` values or the session default.
-    ListSkills {
-        /// Working directories to scope repo skills discovery.
-        ///
-        /// When empty, the session default working directory is used.
-        #[serde(default, skip_serializing_if = "Vec::is_empty")]
-        cwds: Vec<PathBuf>,
-
-        /// When true, recompute skills even if a cached result exists.
-        #[serde(default, skip_serializing_if = "std::ops::Not::not")]
-        force_reload: bool,
-    },
-
     /// Request the agent to summarize the current conversation context.
     /// The agent will use its existing context (either conversation history or previous response id)
     /// to generate a summary which will be returned as an AgentMessage event.
@@ -893,7 +880,6 @@ impl Op {
             Self::ListMcpTools => "list_mcp_tools",
             Self::RefreshMcpServers { .. } => "refresh_mcp_servers",
             Self::ReloadUserConfig => "reload_user_config",
-            Self::ListSkills { .. } => "list_skills",
             Self::Compact => "compact",
             Self::SetThreadMemoryMode { .. } => "set_thread_memory_mode",
             Self::ThreadRollback { .. } => "thread_rollback",
@@ -1441,9 +1427,6 @@ pub enum EventMsg {
     /// List of MCP tools available to the agent.
     McpListToolsResponse(McpListToolsResponseEvent),
 
-    /// List of skills available to the agent.
-    ListSkillsResponse(ListSkillsResponseEvent),
-
     /// List of voices supported by realtime conversation streams.
     RealtimeConversationListVoicesResponse(RealtimeConversationListVoicesResponseEvent),
 
@@ -3312,12 +3295,6 @@ impl fmt::Display for McpAuthStatus {
     }
 }
 
-/// Response payload for `Op::ListSkills`.
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct ListSkillsResponseEvent {
-    pub skills: Vec<SkillsListEntry>,
-}
-
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
 pub struct RealtimeConversationListVoicesResponseEvent {
     pub voices: RealtimeVoicesList,
@@ -3427,19 +3404,6 @@ pub struct SkillToolDependency {
     pub url: Option<String>,
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct SkillErrorInfo {
-    pub path: PathBuf,
-    pub message: String,
-}
-
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct SkillsListEntry {
-    pub cwd: PathBuf,
-    pub skills: Vec<SkillMetadata>,
-    pub errors: Vec<SkillErrorInfo>,
-}
-
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS, PartialEq, Eq)]
 pub struct SessionNetworkProxyRuntime {
     pub http_addr: String,