fix(opencode): add mode: subagent to generated agent frontmatter

Without mode: subagent, all 68 agents default to mode: all and appear as
primary agents the user Tab-cycles through, making the UX unusable.
With mode: subagent they are invoked on-demand via @agent-name.

scripts/convert.sh

@@ -169,6 +169,7 @@ convert_opencode() {
 ---
 name: ${name}
 description: ${description}
+mode: subagent
 color: ${color}
 ---
 ${body}