Add zarvis reset slash command

[edwin-zvs]

Summary

• add a zarvis /reset slash command that clears in-memory model context, queued input, and editor history
• clear zarvis persisted conversation context on reset
• add a reset session event so the daemon truncates transcript and PTY replay history, clears task state, and notifies TUI clients

Verification

• cargo check -p agentd-protocol -p agentd -p agentd-cli -p agentd-adapter-zarvis
• cargo test -p agentd-adapter-zarvis
• cargo test -p agentd-protocol
• cargo build -p agentd -p agentd-cli -p agentd-adapter-zarvis

Note: cargo test -p agentd-cli still has the pre-existing timing failure in pty_render::tests::many_pinned_sessions_resize_within_budget (52ms vs 50ms threshold); all other CLI tests passed.