/clear should reset permission mode to defaultMode

Problem

When defaultMode: "plan" is set in ~/.claude/settings.json, it only applies at session start. Running /clear preserves the current mode instead of re-applying defaultMode.

Expected: /clear resets to defaultMode (e.g., back to plan mode).
Actual: /clear stays in whatever mode was active (e.g., if user exited plan mode, /clear stays in default mode).

Why

/clear is effectively a conversation reset — users expect it to restore initial state, including permission mode. Currently there's no hook or setting to achieve this behavior either (Stop hook can't change modes, SessionStart doesn't fire on /clear).

Suggested fix

After /clear, re-apply permissions.defaultMode from settings.