product: session deep-links — make session/workspace selection URL-addressable

[anantheparty]

Proposal (from a zero-context external scan)

Make session/workspace selection addressable in the URL (?s=<sessionId> via history.replaceState, plus restore-on-load).

Why

• The product's core pitch is supervising parallel background turns, but selection lives only in sessionStorage (apps/web/src/app/App.tsx:143-182) — two windows cannot watch two sessions, a watched session cannot be bookmarked, and closing the tab loses the place entirely (README L73-80 acknowledges this).
• Multi-window/multi-agent supervision is the direction the ecosystem is converging on (OpenAI Codex app, Cursor 2.0 multi-agent views).
• AGENTS.md already scopes browser state to "drafts, focus, selection, expansion, and connection preferences" — the URL is the natural navigation-reference layer for selection, without moving any server-owned state.

Scope sketch

• App.tsx:805 (activeSessionKey) as the source; history.replaceState on selection change, parse-on-boot to restore.
• Encoding only (session id), no tokens/origin in the URL — connection credentials stay tab-scoped per the existing security posture.
• Deliberately replaceState (no history spam); bookmarking one session is the goal.

[anantheparty]

进度更新（v1 已实现）：#51 合并——选择写 ?s=（replaceState）、boot 消费、stale 剥离，e2e 守护。v1 边界：多窗口仲裁与跨机器恢复刻意未做（需服务端会话目录），见跨 tab 所有权提案（即将立案）。