* fix(app): move keyboard focus to target pane on keyboard navigation (#137)
Add a pane focus registry so each panel type can expose its own focus
callback. When keyboard shortcuts navigate between panes, the workspace
screen calls the target panel's registered focus function, moving DOM
focus to the correct input element.
- Agent panels register a callback that focuses the message input
- Terminal panels register a callback that triggers terminal focus
- Panel-internal focus logic stays inside each panel type
- No DOM selectors or panel knowledge in shared infrastructure
* fix(app): simplify workspace pane focus handoff
Replace hardcoded renderContent() switch and per-kind descriptor logic
with a registry-based panel interface. Each panel type (agent, draft,
terminal, file) now self-registers with its component, useDescriptor
hook, and optional confirmClose. Panels access workspace context via
usePaneContext() instead of prop drilling. This prepares the workspace
screen for split pane support.
