Removed workspace header

The old top header bar has been removed from the app shell.

Buttons that were present

Project switcher
- Show the current project name.
- Planned action: switch between projects.
Thread actions
- Overflow menu button.
- Planned action: open thread-level actions.
Run action
- Play button.
- Planned action: configure or trigger a thread-specific run action.
Open
- Folder button.
- Planned action: open workspace/project resources.
Open options
- Chevron next to Open.
- Planned action: show alternate open targets or open-related menu options.
Handoff
- Arrow-left-right button.
- Planned action: hand work off somewhere else, likely another environment, thread, or collaborator flow.
Workspace popout
- External-window button.
- Planned action: open the workspace in a separate popout window.

The header looked mostly mock/planned rather than fully implemented.
Several controls were status-tagged as mock in the codebase.
Commit-related actions appear to have moved into the composer git surface instead of living in the top header.