Add CustomMessageEntry rendering infrastructure

- Add renderCustomMessage to HookAPI for registering custom renderers
- Add CustomMessageRenderer type and CustomMessageRenderOptions
- Store customMessageRenderers in LoadedHook
- Add getCustomMessageRenderer(customType) to HookRunner
- SessionContext.entries now aligned with messages (same length, corresponding indices)

TUI can now correlate messages with their source entries to identify
custom_message entries and use hook-provided renderers.

packages/coding-agent/src/core/hooks/index.ts

@@ -5,6 +5,8 @@ export type {
 	AgentEndEvent,
 	AgentStartEvent,
 	BashToolResultEvent,
+	CustomMessageRenderer,
+	CustomMessageRenderOptions,
 	CustomToolResultEvent,
 	EditToolResultEvent,
 	ExecResult,