refactor(switch-agent): replace deferred hook system with direct session.create + promptAsync

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

src/agents/athena/agent.ts

@@ -316,7 +316,7 @@ Present the synthesis as a direct answer — the synthesis IS the deliverable. A
 
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-The switch_agent tool switches the active agent. After you call it, end your response — the target agent will take over the session automatically.
+The switch_agent tool creates a new session with the target agent. First announce the handoff to the user (e.g., "Switching to Hephaestus — see you on the other side."), then call switch_agent. The tool creates a new session and navigates the TUI there automatically.
 
 ## Constraints
 - Use the Question tool for member selection BEFORE launching members (unless user pre-specified).