External agent session support (#19895)

## Summary

This extends external agent detection/import beyond config artifacts so
Codex can detect recent sessions files from the external agent home and
import them into Codex rollout history.

## What changed

- Added a focused `external_agent_sessions` module for:
  - session discovery
  - source-record parsing
  - rollout construction
  - import ledger tracking
- Wired session detection/import into the app-server external agent
config API.
- Added compaction handling so large imported sessions can be resumed
safely before the first follow-up turn.

## Testing

Added coverage for:
- recent-session detection
- custom-title handling
- recency filtering
- dedupe and re-detect-after-source-change behavior
- visible imported turn construction
- backward-compatible import payload deserialization
- end-to-end RPC import flow
- rejection of undetected session paths
- repeat-import behavior
- large-session compaction before first follow-up

Ran:
- `cargo test -p codex-app-server external_agent_config_import_ --test
all`

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -1091,6 +1091,9 @@ pub enum ExternalAgentConfigMigrationItemType {
     #[serde(rename = "MCP_SERVER_CONFIG")]
     #[ts(rename = "MCP_SERVER_CONFIG")]
     McpServerConfig,
+    #[serde(rename = "SESSIONS")]
+    #[ts(rename = "SESSIONS")]
+    Sessions,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
@@ -1105,11 +1108,23 @@ pub struct PluginsMigration {
     pub plugin_names: Vec<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct SessionMigration {
+    pub path: PathBuf,
+    pub cwd: PathBuf,
+    pub title: Option<String>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct MigrationDetails {
+    #[serde(default)]
     pub plugins: Vec<PluginsMigration>,
+    #[serde(default)]
+    pub sessions: Vec<SessionMigration>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
@@ -7841,11 +7856,50 @@ mod tests {
                         marketplace_name: "team-marketplace".to_string(),
                         plugin_names: vec!["asana".to_string()],
                     }],
+                    sessions: Vec::new(),
                 }),
             }
         );
     }
 
+    #[test]
+    fn external_agent_config_import_params_accept_legacy_plugin_details() {
+        let params: ExternalAgentConfigImportParams = serde_json::from_value(json!({
+            "migrationItems": [{
+                "itemType": "PLUGINS",
+                "description": "Install supported plugins from Claude settings",
+                "cwd": absolute_path_string("repo"),
+                "details": {
+                    "plugins": [
+                        {
+                            "marketplaceName": "team-marketplace",
+                            "pluginNames": ["asana"]
+                        }
+                    ]
+                }
+            }]
+        }))
+        .expect("legacy plugin import params should deserialize");
+
+        assert_eq!(
+            params,
+            ExternalAgentConfigImportParams {
+                migration_items: vec![ExternalAgentConfigMigrationItem {
+                    item_type: ExternalAgentConfigMigrationItemType::Plugins,
+                    description: "Install supported plugins from Claude settings".to_string(),
+                    cwd: Some(PathBuf::from(absolute_path_string("repo"))),
+                    details: Some(MigrationDetails {
+                        plugins: vec![PluginsMigration {
+                            marketplace_name: "team-marketplace".to_string(),
+                            plugin_names: vec!["asana".to_string()],
+                        }],
+                        sessions: Vec::new(),
+                    }),
+                }],
+            }
+        );
+    }
+
     #[test]
     fn command_execution_request_approval_rejects_relative_additional_permission_paths() {
         let err = serde_json::from_value::<CommandExecutionRequestApprovalParams>(json!({