Hook commands registered via pi.registerCommand() are now handled in
AgentSession.prompt() alongside file-based slash commands. This:
- Removes duplicate tryHandleHookCommand from interactive-mode and rpc-mode
- All modes (interactive, RPC, print) share the same command handling logic
- AgentSession._tryExecuteHookCommand() builds CommandContext using:
- UI context from hookRunner (set by mode)
- sessionManager, modelRegistry from AgentSession
- sendMessage via sendHookMessage
- exec via exported execCommand
- Handler returning string uses it as prompt, undefined returns early
Also:
- Export execCommand from hooks/runner.ts
- Add getUIContext() and getHasUI() to HookRunner
- Make HookRunner.emitError() public for error reporting
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