fix(desktop): keep agent activity feed pinned to bottom across smooth-scroll

[tlongwell-block]

What

Fix the activity / observer feed "jump back a couple lines" bug reported by Will.

useStickToBottom (used by AgentSessionThreadPanel) recomputed isNearBottomRef from raw scrollTop on every scroll event. When the hook scheduled a behavior: "smooth" scroll-to-bottom to follow streaming agent content, the browser emitted intermediate scroll events whose scrollTop was mid-animation — distance > 100 even though the viewport was animating toward the bottom. The buggy handler flipped isNearBottom to false during those events, and the next content append refused to follow.

User-visible symptom: scroll all the way down while an agent is working, wait a few seconds, and the view jumps back a couple lines.

Fix

Make onScroll direction-aware via lastScrollTopRef:

• distance < 100 → always sticky (handles initial state and re-sticks once an animation settles)
• scrollTop < lastScrollTop → user pulled the viewport upward, detach
• Otherwise → leave the sticky bit alone (smooth-scroll in flight toward the bottom)

The initial-scroll effect seeds lastScrollTopRef so the first real scroll event has a meaningful prior value.

~10 net lines in useStickToBottom.ts. No behavior change for the user-pulled-up path.

Tests

Two Playwright specs in tests/e2e/stick-to-bottom-scroll-jump.spec.ts drive a tiny React fixture (src/testing/stickToBottomFixture.tsx) that uses the real hook in a scroll container, plus a small simulateScroll(top) method to deterministically inject the intermediate scroll event that production sees mid-animation. The fixture mounts via the existing e2e bridge with a lazy-installed install hook so the production bundle isn't bloated.

1. intermediate smooth-scroll events do not unstick the activity feed — pins the regression. Grows the scroll range with a content burst, then dispatches a scroll event whose scrollTop is climbing toward the new bottom but distance > 100. Pre-fix this assertion fails on isNearBottom; post-fix it passes. Verified by reverting the hook locally — that single spec fails on exactly expect(midAnimation.isNearBottom).toBe(true).
2. user scrolling up still unsticks the activity feed — guards against over-correction. Passes both with and without the fix; ensures the direction-aware logic still releases when the user pulls the view up.

Registered in the smoke project.

Verification

• pnpm typecheck ✓
• pnpm check ✓
• pnpm build ✓
• pnpm exec playwright test stick-to-bottom-scroll-jump.spec.ts --project=smoke → 2 passed
• Full smoke suite re-run on the 4 unrelated flakes (custom-emoji, messaging, spoiler, video-attachment) → 46/46 pass in isolation; not caused by this change

Reported by @willpfleger.