[Feature Request] Scope cross-session communication to opt-in parent-child links; disable by default between peer sessions

**Bug Description**
Cross-session communication for multiple sessions and multiple agents, all working in different terminal tabs simultaneously, each session on a different feature or different module — they communicate with each other and cause issues in each other's contexts! I'm starting to get lost. I hope it's disabled by default and only enabled if needed when a sub-agent is deployed as a session, so the parent agent can communicate with the sub-agent! But don't allow it between main/parent and main/parent sessions by default!

**Environment Info**
- Platform: linux
- Terminal: vscode
- Version: 2.1.272
- Feedback ID: 57a1fe3d-b37c-4bde-b7ba-ccbd77d4fe75

**Errors**
```json
[]
```

No implementation files, tests, or entry points are named. First reproduce the cross-session communication behavior using multiple Linux terminal tabs in VS Code, then trace how sessions and parent-child relationships are identified; done means peer sessions are isolated by default while explicitly linked parent-child sessions can communicate.

Written by the indexing model from the issue text.