Agents Window: Left-align capability pills above the input

- Feature (e.g. agent/edit/ask mode): Agents window
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): N/A
- Logs: N/A

Steps to Reproduce:

1. Open the Agents window.
2. Display a capability pill above the input box, such as a subagent or browser pill.
3. Observe its horizontal alignment.

Actual behavior:
The pill is centered above the input box.

Expected behavior:
The pill should be left-aligned with the input area. The alignment should be consistent for all pills shown in this location.