refactor(cli): improve send-message hook architecture (Commit 2.1)

Extracts and reorganizes the send-message hook into focused modules:
- use-message-execution.ts: Core SDK execution with agent resolution
- use-run-state-persistence.ts: Run state management and persistence
- helpers/send-message.ts: Helper functions for message preparation and error handling

Fixes out-of-credits (402) error handling regression:
- Preserves statusCode in ExecuteMessageError for HTTP errors
- Detects 402 errors in the non-throwing path and handles them properly
- Ensures UI switches to out-of-credits mode when credits are exhausted

Author: brandonkachen