[T2.2] Harden optimistic send and auto-follow contract

[DeliciousBuding]

Task: T2.2 - Harden optimistic send and auto-follow contract

Phase: 2 - Shared Chat Timeline Hardening
Priority: P0 | Size: M | Lane: A
S.U.P.E.R Drivers: U,R
Test Expectation: Shared auto-scroll tests plus Desktop/Web E2E
Memory/Governance Impact: None

Description

Make local user sends appear immediately, avoid flash/disappear during refetch, and follow scroll after submit.

Acceptance Criteria

• Submitted user message appears immediately
• Message does not disappear after first visible paint
• Auto-follow works after submit while user scrollback is respected
• Passes S.U.P.E.R Quick Check for: U,R
• Satisfies test expectation: Shared auto-scroll tests plus Desktop/Web E2E
• Updates resolved memory or instruction surfaces if durable project knowledge or agent instructions changed

Affected Files

• app/shared/src/chatview/
• app/desktop/src/
• app/web/src/

Dependencies

• Depends on: See docs/plan/dependency-graph.md

---

Managed by Spec-Driven Develop workflow