Add manual refresh

[tnez]

Context
V1 refresh is manual-only. Users need keyboard paths to refresh focused panel and all panels. See docs/v1-scope.md.

Acceptance criteria

• User can refresh the focused panel.
• User can refresh all panels.
• Refresh updates panel state through loading, ready, or error outcomes.
• Refresh behavior is testable without real shell commands.
• Keybindings are documented.

Out of scope

• Per-panel intervals.
• Background polling.
• Long-running command supervision.

[tnez]

Implementation is complete in PR #11: #11. I could not close this issue from here because merging PR #11 into main was blocked pending explicit approval for the default-branch mutation. The PR is green and links this issue for automatic closure on merge.