#9103: feat(message-tool): add responsePrefix support for agent identification

## Summary Adds responsePrefix template support to the message tool so proactive sends include the agent identity prefix (e.g., `[AgentName]`). ## Problem - Agent replies through auto-reply flow get `[{identityName}]` prefix ✅ - Proactive sends via message tool don't get prefix ❌ - Users can't tell which agent sent proactive messages ## Solution Apply `responsePrefix` in `message-action-runner.ts` after `maybeApplyCrossContextMarker`: - Resolve `responsePrefix` from config - Resolve `identityName` for current agent - Prepend to message if not already present ## Changes - `src/infra/outbound/message-action-runner.ts`: Apply responsePrefix after crossContextMarker - `src/tui/`: Add agent name labels to assistant messages + lint fixes ## Testing - ✅ Build passes - ✅ Lint passes ## Configuration Requires `messages.responsePrefix` and `agents.list[].identity.name` in config.