Add --json flag to codex exec for structured output (#12)

## Summary
- Adds `--json` flag to `codex exec` command in the build output
- Suppresses verbose prompt echo, outputs structured JSONL events
instead

## Test plan
- [ ] Run a session via strawpot-codex and verify prompt is no longer
echoed in output
- [ ] Verify JSONL events are emitted on stdout

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: juhgiyo

codex/wrapper/main.go

@@ -231,6 +231,9 @@ func cmdBuild(args []string) {
 	// Build codex command
 	cmd := []string{"codex", "exec"}
 
+	// JSON output for machine parsing (suppresses prompt echo)
+	cmd = append(cmd, "--json")
+
 	if ba.Task != "" {
 		cmd = append(cmd, ba.Task)
 	}

codex/wrapper/main_test.go

@@ -134,9 +134,9 @@ func TestCmdBuild_WithTaskAndModel(t *testing.T) {
 
 	cmd := result["cmd"].([]interface{})
 
-	// Task is positional after "codex exec"
-	if len(cmd) < 3 || cmd[2] != "fix the bug" {
-		t.Errorf("cmd[2] = %v, want %q", cmd[2], "fix the bug")
+	// Task is positional after "codex exec --json"
+	if len(cmd) < 4 || cmd[3] != "fix the bug" {
+		t.Errorf("cmd[3] = %v, want %q", cmd[3], "fix the bug")
 	}
 	assertSequence(t, cmd, "-m", "gpt-5.2-codex")
 }