Add CustomMessageEntry for hook-injected messages in LLM context

- CustomMessageEntry<T> type with customType, content, display, details
- appendCustomMessageEntry() in SessionManager
- buildSessionContext() includes custom_message entries as user messages
- Exported CustomEntry and CustomMessageEntry from main index

CustomEntry is for hook state (not in context).
CustomMessageEntry is for hook-injected content (in context).

packages/coding-agent/src/index.ts

@@ -111,6 +111,8 @@ export {
 	buildSessionContext,
 	type CompactionEntry,
 	CURRENT_SESSION_VERSION,
+	type CustomEntry,
+	type CustomMessageEntry,
 	createSummaryMessage,
 	type FileEntry,
 	getLatestCompactionEntry,