tui: plan mode silently not enforced after re-engaging mid-session — agent executes mutations without restriction

Summary

In the v2 beta TUI, re-engaging plan mode mid-session (after previously
exiting it) sometimes fails to inject the plan-mode system reminder into the
conversation. The UI indicates plan mode is active, but the agent executes
file mutations without restriction. Observed in a session where an earlier
plan-mode engagement DID inject the reminder correctly.

Environment

• opencode version: 0.0.0-beta-17759
• OS: macOS 26 (Darwin 25.5.0, arm64)
• Terminal: tmux (xterm-256color, truecolor)
• Shell: /bin/zsh
• Install/channel: beta (mise npm: opencode-ai@beta)
• Active plugins: opencode-buddy (v1-syntax plugin entry),
  plugins/switchboard.ts, plugins/worktrunk.ts

Reproduction

1. Launch opencode2, work through several agent/mode switches where plan
   mode engages and disengages normally (reminder injected each time)
2. Later, re-engage plan mode via agent switch
3. UI shows plan mode active; no plan-mode reminder reaches the model
4. Instruct the agent to mutate a file — it executes (rm of a plugin file
   observed) with no restriction message

Expected Behavior

Every plan-mode engagement injects the plan-mode system reminder and enforces
read-only behavior, regardless of how many prior mode transitions occurred in
the session.

Actual Behavior

Re-engagement after prior switches silently skipped reminder injection.
Agent remained fully mutable. No error or indication of the missed injection;
only user-side observation of unauthorized mutations revealed it.

Additional Context

• Earlier plan-mode engagements in the same long session injected the
  reminder correctly, so the failure is intermittent/state-dependent
• Session included custom primary agents (~/.config/opencode/agent/*.md)
  alongside built-in plan/build
• Possibly related to plugin event handling (v1-syntax buddy entry +
  switchboard.ts were loaded)