fix(pi): strip planning-only tools on fresh idle sessions

[dmmulroy]

Problem

PR #446 refactored the session_start handler to use applyPhaseConfig() but changed the idle branch to only call restoreSavedState(), which no-ops when savedState is null (i.e., fresh sessions that were never in planning mode). This left plannotator_submit_plan in the active tool set even though plan mode hadn't been activated — a regression from the fix in #387.

Root Cause

In the session_start handler, the idle branch was:

if (phase === "idle") {
    await restoreSavedState(ctx);  // ← no-ops when savedState is null
    savedState = null;
}

When savedState is null (fresh session, never entered planning), restoreSavedState() returns immediately at line 204 without calling setActiveTools(), so plannotator_submit_plan is never stripped.

Fix

The idle branch now explicitly strips planning-only tools when there is no saved state to restore:

if (phase === "idle") {
    if (savedState) {
        await restoreSavedState(ctx);
        savedState = null;
    } else {
        pi.setActiveTools(stripPlanningOnlyTools(pi.getActiveTools()));
    }
}

This restores the guarantee from #387 that plannotator_submit_plan is always removed from the active tool set when not in planning mode.