Fix executor switch: kill old session and restart with new executor

[bborn]

Summary

• When switching executors on a task in the TUI, the old tmux window was being reused (same Claude pane, just with a new title). Now Refresh() detects executor changes and kills the old window, clears stale state, and starts a fresh session with the new executor.
• Exports KillAllWindowsByNameAllSessions for use from the UI package.

What was happening

1. User edits task, changes executor from Claude to Codex
2. startResumableSession finds existing task-{ID} window and reuses it
3. Pane title updates to "Codex" but the running process is still Claude

What happens now

1. Refresh() detects prevTask.Executor != m.task.Executor
2. Kills the old tmux window
3. Clears cached pane IDs and stale session ID
4. Starts fresh session with new executor (including previous session context from Preserve session content when switching executors #511)

Test plan

• All executor and UI tests pass
• go vet and golangci-lint clean
• Manual: switch executor on running task, verify old pane is killed and new executor starts

🤖 Generated with Claude Code