feat(cli): add agent_message type for precise --final output filtering

Introduce dedicated agent_message IR type to distinguish final AI responses
from generic stdout. This enables --final flag to show only agent messages,
filtering out all intermediate content (JSONL events, reasoning, tool calls).

Changes:
- Add agent_message type to CliOutputUnitType
- Update JsonLinesParser to map final responses from all tools (codex,
  gemini, claude, opencode) to agent_message type
- Add final_output field to database schema with migration
- Update getCachedOutput and getConversation to return finalOutput
- Prefer finalOutput in outputAction for --final flag

Fixes issue where --final showed raw JSONL instead of filtered content.

ccw/src/tools/cli-executor-state.ts

@@ -85,6 +85,7 @@ export interface ExecutionRecord {
     truncated: boolean;
   };
   parsedOutput?: string;  // Extracted clean text from structured output units
+  finalOutput?: string;  // Agent message only (for --final flag)
 }
 
 interface HistoryIndex {
@@ -109,6 +110,7 @@ export interface ExecutionOutput {
   stdout: string;
   stderr: string;
   parsedOutput?: string;  // Extracted text from stream JSON response
+  finalOutput?: string;  // Agent message only (for --final flag)
 }
 
 /**