feat: add footerShowSubagents config option for TUI agent display

[aadilshaikh123]

Add a new TUI configuration option to control whether subagents are shown in the footer agent tab selector.

When set to false (default), only primary agents are displayed, providing a cleaner workflow for users with many agents from plugins. When set to true, all non-hidden agents (including subagents) are displayed.

This allows users with heavy multi-agent setups to restore the previous primary-only visibility while keeping the new subagent orchestration features for those who want full visibility.

Fixes: #12530, #4764

### Issue for this PR

Closes #12530, #4764

### Type of change

- [x] New feature
- [ ] Bug fix
- [ ] Refactor / code improvement
- [ ] Documentation

### What does this PR do?

Add a new TUI configuration option `footerShowSubagents` to control whether subagents are shown in the footer agent tab selector.

**Problem:** Since v1.3.4, the footer agent display includes all agents (primary + subagents), making manual switching slow for users with many custom agents and plugins like compound-engineering.

**Solution:** Add a configuration option (default: false) to restore the previous clean primary-only agent display while keeping the new subagent orchestration features available for those who want full visibility.

Users can now configure this in tui.json:
```json
{
  "footerShowSubagents": false
}

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