Move external agent migration into its crate (#33456)

## What changed

- Move migration detection, import orchestration, models, and reporting from
  `codex-app-server` into `codex-external-agent-migration`.
- Keep app-server protocol conversion in a dedicated boundary module.
- Organize source-specific configuration, sessions, hooks, plugins, commands,
  subagents, and memory handling behind shared migration adapters.
- Relocate and split the migration tests alongside the extracted service.

GitOrigin-RevId: 3378ec2064806a28136d6ac98664ca1d75414a4e

codex-rs/external-agent-migration/src/model.rs

@@ -0,0 +1,139 @@
+use crate::sessions::ExternalAgentSessionMigration;
+use std::path::PathBuf;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ExternalAgentConfigDetectOptions {
+    pub include_home: bool,
+    pub include_memory: bool,
+    pub cwds: Option<Vec<PathBuf>>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ExternalAgentConfigMigrationItemType {
+    Config,
+    Skills,
+    AgentsMd,
+    Plugins,
+    McpServerConfig,
+    Subagents,
+    Hooks,
+    Commands,
+    Memory,
+    Sessions,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PluginsMigration {
+    pub marketplace_name: String,
+    pub plugin_names: Vec<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct NamedMigration {
+    pub name: String,
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct MigrationDetails {
+    pub plugins: Vec<PluginsMigration>,
+    pub skills: Vec<NamedMigration>,
+    pub sessions: Vec<ExternalAgentSessionMigration>,
+    pub mcp_servers: Vec<NamedMigration>,
+    pub hooks: Vec<NamedMigration>,
+    pub subagents: Vec<NamedMigration>,
+    pub commands: Vec<NamedMigration>,
+    pub memory: Vec<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PendingPluginImport {
+    pub cwd: Option<PathBuf>,
+    pub description: String,
+    pub details: MigrationDetails,
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct PluginImportOutcome {
+    pub succeeded_marketplaces: Vec<String>,
+    pub succeeded_plugin_ids: Vec<String>,
+    pub failed_marketplaces: Vec<String>,
+    pub failed_plugin_ids: Vec<String>,
+    pub raw_errors: Vec<ExternalAgentConfigImportRawError>,
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct ExternalAgentConfigImportOutcome {
+    pub pending_plugin_imports: Vec<PendingPluginImport>,
+    pub item_results: Vec<ExternalAgentConfigImportItemResult>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ExternalAgentConfigImportItemResult {
+    pub item_type: ExternalAgentConfigMigrationItemType,
+    pub description: String,
+    pub cwd: Option<PathBuf>,
+    pub success_count: u32,
+    pub error_count: u32,
+    pub successes: Vec<ExternalAgentConfigImportSuccess>,
+    pub raw_errors: Vec<ExternalAgentConfigImportRawError>,
+}
+
+impl ExternalAgentConfigImportItemResult {
+    pub fn new(
+        item_type: ExternalAgentConfigMigrationItemType,
+        description: String,
+        cwd: Option<PathBuf>,
+    ) -> Self {
+        Self {
+            item_type,
+            description,
+            cwd,
+            success_count: 0,
+            error_count: 0,
+            successes: Vec::new(),
+            raw_errors: Vec::new(),
+        }
+    }
+
+    pub fn record_error(&mut self, raw_error: ExternalAgentConfigImportRawError) {
+        self.error_count = self.error_count.saturating_add(1);
+        self.raw_errors.push(raw_error);
+    }
+
+    pub fn record_success(&mut self, source: Option<String>, target: Option<String>) {
+        self.success_count = self.success_count.saturating_add(1);
+        self.successes.push(ExternalAgentConfigImportSuccess {
+            item_type: self.item_type,
+            cwd: self.cwd.clone(),
+            source,
+            target,
+        });
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ExternalAgentConfigImportSuccess {
+    pub item_type: ExternalAgentConfigMigrationItemType,
+    pub cwd: Option<PathBuf>,
+    pub source: Option<String>,
+    pub target: Option<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ExternalAgentConfigImportRawError {
+    pub item_type: ExternalAgentConfigMigrationItemType,
+    pub error_type: Option<String>,
+    pub sub_error_type: Option<String>,
+    pub failure_stage: String,
+    pub message: String,
+    pub cwd: Option<PathBuf>,
+    pub source: Option<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ExternalAgentConfigMigrationItem {
+    pub item_type: ExternalAgentConfigMigrationItemType,
+    pub description: String,
+    pub cwd: Option<PathBuf>,
+    pub details: Option<MigrationDetails>,
+}