fix(desktop): reset telemetry and context panel after /clear

[ashishexee]

Closes #4186

Problem

After /clear, the context panel still shows stale token counts and workspace context.

Two sub-bugs:

• (a) /clear sometimes fails to clear context — stale usage events from the previous session re-populate state after the reset action zeros it.
• (b) Token info and workspace context are not refreshed after /clear.

Root Causes

1. ClearSession() resets the executor session but NOT tab.usageTelemetry/tab.readTelemetry. ContextPanel reads from telemetry, returns stale data.
2. dockRefreshKey only bumps on turn_done, so ContextPanel doesn't refresh after /clear.
3. ContextPanel.tsx line 212 falls back to info.usedTokens when context.used === 0, showing stale values.
4. The usage event handler doesn't check turnActive, so stale events arriving after reset re-accumulate into session tokens.

Fix

1. Add resetTelemetry() on WorkspaceTab — called from App.ClearSession() after clearing the executor session.
2. Bump dockRefreshKey in confirmClearContext to force ContextPanel refresh.
3. Clear stale info state in ContextPanel when context.used === 0 && sessionTokens === 0.
4. Guard usage handler with turnActive check — stale events from a cleared session are dropped.
5. Add sessionGen counter to State, incremented on reset — for future use to discard stale events by generation.

Files Changed

• desktop/tabs.go — resetTelemetry() method
• desktop/app.go — calls tab.resetTelemetry() after ClearSession()
• desktop/frontend/src/App.tsx — bumps dockRefreshKey after clear
• desktop/frontend/src/components/ContextPanel.tsx — clears stale info
• desktop/frontend/src/lib/useController.ts — sessionGen counter, usage guard on turnActive

Verification

go build ./cmd/reasonix
go vet ./...
go test ./internal/control/ -run TestSubmitClear -v -count=1
go test ./internal/agent/ -count=1
go test ./internal/config/ -count=1