- 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.
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