feat(chat): autoscroll, sub-agent auto-open, jump-to-bottom button

[BatLeDev]

Move the chat transcript's autoscroll into the shared useAutoScrollBottom composable (@data-fair/lib-vue), make sub-agent panel auto-open deterministic, and add a jump-to-bottom button.

• Autoscroll now lives in AgentChatMessages via the shared composable; the parent-driven scroll watchers (and messagesRef/defineExpose) are removed from AgentChat.vue.
• The growth signal (streamedLength) counts parent text, tool-invocation chips, and nested sub-agent growth, so following keeps up while a sub-agent — not the parent message — is streaming.
• Sub-agent panels hold per-message open state: while following, the active message auto-opens its latest panel (a newer sub-agent collapses the previous one); once the user scrolls up, panel state is left untouched.
• A jump-to-bottom button appears whenever the container isn't at the bottom. atBottom is tracked on scroll, on content growth, and via a ResizeObserver, so it stays correct when content height shrinks (a panel collapsing, a turn ending) without firing a scroll event.
• Extracts pure helpers into auto-scroll.ts with 11 unit tests, hardens the e2e to assert auto-open/auto-close, and bumps @data-fair/lib-vue to ^1.28.1 (position-based stick-to-bottom fix; it adds an optional @vueuse/core peer, which is what re-marks the peer: true churn in package-lock.json).

Why: follow the streaming tail reliably (the old last-message-length signal froze during sub-agent delegation, and panel auto-open was flaky because it was gated on scroll state), and share the composable with processings' run log.

Regression risks:

• Scroll behavior change: the transcript no longer force-scrolls to the bottom on every new message/content — it follows only while the user is at the bottom. Someone who scrolled up to read history is no longer yanked down.
• AgentChat.vue no longer owns scroll logic or exposes messagesContainer — verified no other consumer referenced the ref.
• openPanels is keyed by message index and only cleared while following is true; if the user is scrolled up when a new conversation loads, manually-opened panel state can carry over by index. Low impact.
• New ResizeObserver on the scroll container and its content (disconnected on unmount) — double-check it doesn't fire excessively on very large transcripts.