feat(web): replace sessions table with workbench

[ZeroPointSix]

Review Contract

Goal

Replace the legacy ProTable-based /sessions and /sessions/:id views with a three-column, read-only Session Workbench that preserves the existing React Query, SSE, authentication, and HashRouter behavior while making session observation the primary user experience.

Non-goals

This change does not implement transcript data or the W4 Activity Feed, agent switching, New Agent creation, New chat behavior, model switching, session mutation, Stop/Cancel controls, or any backend API changes.

Accepted Residual Risks

The middle panel continues to render the existing session status event timeline until W4 supplies the Activity Feed. Ant Design X is pinned to the 1.6.1 line because the current application uses Ant Design 5; upgrading to Ant Design X 2.x is deferred until the application upgrades to Ant Design 6. The local browser smoke test uses a non-sensitive in-memory mock API because no Gateway backend is part of this checkout.

Acceptance Criteria

• /sessions and /sessions/:id both render the three-column Workbench, and HashRouter deep links resolve directly to the selected session.
• The left sidebar uses Ant Design X Conversations, groups sessions by Agent/Profile, and provides keyword search, status filtering, session selection, refresh, and a disabled New chat placeholder.
• The main panel shows Agent, Profile, model, status, workdir, and session-link fields, the current status event stream, and a fixed read-only observation bar.
• The Inspector carries the existing metadata, profile_config_errors, last_error, and event timeline in tabs.
• The read-only observation bar exposes the SSE connection states connecting, live, reconnecting, and offline; no write entry, model switch, Stop, or Cancel control is exposed.
• Responsive rules collapse the side panels into Drawers on narrow screens, including the 390px breakpoint.
• SessionTable.tsx, its route references, and the superseded Sessions/SessionDetail page files are removed.
• Verification evidence: pnpm lint, pnpm test (27 tests passed), pnpm build, and a browser smoke test covering /sessions, selecting a session, and /sessions/:id with a local mock API.

Follow-ups

W3 will provide transcript data and W4 will replace the middle status-event stream with the Activity Feed. W6 will own Agent switching, New Agent, and New chat behavior. The PR is ready for maintainer review and contract freeze.

Closes ZER-668

[linear-code]

ZER-668

[ZeroPointSix]

已由 #46 的干净 ZER-404 W1–W5 只读工作台整合替代。#46 从最新 main 按依赖顺序重建，保留本 PR 的有效能力并补齐跨包冲突、活动流接线、恢复链与最新回归修复；原分支保留供历史追溯。