Vim mode: 'k' hotkey kills background agents with no confirmation

Problem

In vim mode, pressing k (which is the standard vim "move up" keybinding) kills a running background agent immediately with no confirmation prompt and no partial output returned.

This is especially painful because:

1. Vim users press k reflexively to navigate — it's muscle memory for "move up"
2. No confirmation dialog — the kill is instant and irreversible
3. No partial output — you lose all the work the agent has done so far. Even if it was 90% done, you get nothing back
4. Background agents can run for minutes — research agents doing web searches can take 5+ minutes, and accidentally killing one means starting over from scratch

Expected behavior

At minimum, one of:

• k in vim mode should move the cursor up, not kill agents
• Kill should require confirmation (e.g., "Agent is running. Kill it? y/n")
• Partial output should be returned when an agent is killed

Reproduction

1. Enable vim mode
2. Launch a background agent (e.g., a research task)
3. While it's running, press k to try to scroll/navigate
4. Agent is immediately killed with no output

Environment

• macOS
• Claude Code CLI with vim mode enabled