[codex] Record external agent import results (#28396)

## Summary
- restore `externalAgentConfig/import/progress` notifications while
keeping `externalAgentConfig/import/completed` as the must-deliver event
- persist completed external-agent config imports in state DB by
`importId`, including concrete success/failure details for config,
AGENTS.md, skills, plugins, MCP servers, subagents, hooks, commands, and
sessions
- add `externalAgentConfig/import/readHistories` so clients can recover
persisted import results after missing the live completion notification
- include `errorType` on import failures in protocol
responses/notifications and persisted DB JSON so future code can
classify failures without another wire/storage shape change

## Validation
- `git diff --check`
- `just test -p codex-state external_agent_config_imports`
- `just test -p codex-app-server-protocol`
- `CODEX_SQLITE_HOME=/private/tmp/codex-app-server-sqlite-read-details
just test -p codex-app-server
external_agent_config_import_sends_completion_notification_for_sync_only_import`

Also ran earlier broader checks before publishing:
- `just test -p codex-state`
-
`CODEX_SQLITE_HOME=/private/tmp/codex-app-server-external-agent-test-sqlite
just test -p codex-app-server external_agent_config`
- `just test -p codex-external-agent-migration`

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

@@ -1103,6 +1103,11 @@ client_request_definitions! {
         serialization: global("config"),
         response: v2::ExternalAgentConfigImportResponse,
     },
+    ExternalAgentConfigImportHistoriesRead => "externalAgentConfig/import/readHistories" {
+        params: #[ts(type = "undefined")] #[serde(skip_serializing_if = "Option::is_none")] Option<()>,
+        serialization: global_shared_read("config"),
+        response: v2::ExternalAgentConfigImportHistoriesReadResponse,
+    },
     ConfigValueWrite => "config/value/write" {
         params: v2::ConfigValueWriteParams,
         serialization: global("config"),
@@ -1614,6 +1619,7 @@ server_notification_definitions! {
     AccountRateLimitsUpdated => "account/rateLimits/updated" (v2::AccountRateLimitsUpdatedNotification),
     AppListUpdated => "app/list/updated" (v2::AppListUpdatedNotification),
     RemoteControlStatusChanged => "remoteControl/status/changed" (v2::RemoteControlStatusChangedNotification),
+    ExternalAgentConfigImportProgress => "externalAgentConfig/import/progress" (v2::ExternalAgentConfigImportProgressNotification),
     ExternalAgentConfigImportCompleted => "externalAgentConfig/import/completed" (v2::ExternalAgentConfigImportCompletedNotification),
     FsChanged => "fs/changed" (v2::FsChangedNotification),
     ReasoningSummaryTextDelta => "item/reasoning/summaryTextDelta" (v2::ReasoningSummaryTextDeltaNotification),