feat: Add chat session subchannels

[0xroylee]

What Changed

• Added shared chat session subchannel helpers and route parsing for /session/:id/chat and /session/:id/task-info.
• Rendered selectable Chat and Task Info child links under each active session in the left sidebar.
• Replaced the chat task-details side sheet with a main-panel Task Info channel that reuses existing React Query task, token usage, and activity hooks.

Why

• Session task details should behave like Slack-style subchannels under each session, with Task Info as an actual selectable channel instead of a separate details panel.
• Durable subchannel URLs make session context linkable and preserve the chatroom mental model.

How to Test

• bun test packages/web/tests/chat-session-subchannels.test.ts packages/web/tests/operator-chat-sidebar-route.test.ts packages/web/tests/chat-room-sidebar-utils.test.ts
• bun run --filter web typecheck
• bun run check
• bun run --filter web build
• bun run typecheck
• bun test
• Browser QA: started web on localhost:3002 and API on 3001 with PGlite, created a session, verified Session -> Chat / Task Info in desktop and mobile sidebar, switched between /chat and /task-info, confirmed the old Details button is gone, and confirmed no browser console warnings/errors.

Additional Notes

• The first sandboxed web build hit the known Turbopack port-binding panic; rerunning the same build outside the sandbox passed.
• The legacy /session/:sessionId route still renders the Chat channel.