[codex] Fix web control-plane connection starvation

[roackb2]

Summary

• Reduce web-v2 session-page EventSource usage to avoid Chrome per-origin connection starvation across multiple Heddle tabs.
• Keep the selected conversation stream live, but replace sidebar session-list streaming with short query refreshes.
• Gate heartbeat event streaming to the Tasks surface and add shared tRPC request timeouts so stalled request/response calls recover.

Root cause

Each Heddle web tab opened several long-lived control-plane streams: selected session events, session-list events, and heartbeat events. With multiple Heddle tabs open, Chrome exhausted the HTTP connection slots for localhost:8765. New tabs or first-load queries then stayed pending until another Heddle tab closed and released a connection.

Behavior change

• Opening another Heddle web tab should load immediately instead of waiting for an existing Heddle tab to close.
• Session pages still stream conversation updates in real time through controlPlane.sessionEvents.
• Sidebar sessions refresh through normal short requests instead of a permanent stream.
• Heartbeat streams are only active while viewing Tasks.
• Hung request/response calls now abort after 30 seconds instead of leaving the composer stuck indefinitely.

Verification

• yarn typecheck
• yarn lint
• yarn client:build:v2
• yarn test:browser-integration:v2