fix(hook): add tool_use/tool_result pair validator to prevent API errors

Adds a defensive tool-pair-validator hook that runs as the final step in
the messages transform pipeline. When compaction or context-window recovery
removes user messages containing tool_result blocks without removing the
preceding assistant message with tool_use blocks, this validator detects
the mismatch and either:

1. Injects missing tool_result parts into the next user message, or
2. Creates a synthetic user message with placeholder tool_results

This prevents Anthropic API errors like 'tool_use ids found without
tool_result blocks immediately after'.

Fixes #3014

src/config/schema/hooks.ts

@@ -25,6 +25,7 @@ export const HookNameSchema = z.enum([
   "interactive-bash-session",
 
   "thinking-block-validator",
+  "tool-pair-validator",
   "ralph-loop",
   "category-skill-reminder",