Adopt the opencode-handoff pattern: switch_agent now creates a new session
and sends the context via promptAsync immediately in the tool's execute
function, then navigates the TUI to the new session.
Removes ~1700 LOC of complexity: persistent state management, event-based
hook monitoring, retry logic, session idle waiting, apply verification,
and terminal detection — all replaced by 3 SDK calls.
Deleted: src/features/agent-switch/ (14 files), src/hooks/agent-switch/ (4 files)
Updated: Athena prompt to announce handoff before calling switch_agent
ismeth