Feature Request: Allow filtering/hiding chat sessions by specific agent/provider

## Summary

Currently, when multiple agent providers (Copilot, Claude, Codex, etc.) are active, their chat sessions are all merged into the same session list (Chat view / Agent Sessions), with no way to hide sessions from one specific provider while keeping others visible.

## Use Case

I use Claude Code and the OpenAI Codex extension side by side in VS Code. I want to:
- Keep using the Codex extension for occasional tasks (via its own panel/sidebar)
- Prevent Codex's sessions from cluttering the main Chat view / session list, which I primarily use for Claude/Copilot sessions

Currently there is no setting to achieve this. Related attempts that don't fully work:
- `chat.agentHost.enabled: false` doesn't fully hide sessions from that provider (see #318836)
- Archiving/deleting sessions only clears existing history; new sessions from the unwanted provider still appear afterward
- Dragging a provider's tab out to its own sidebar panel (as a workaround) doesn't stop its sessions from also appearing in the main Chat view's session list

This is somewhat related to #277696, which asks that hidden agents remain usable — my request is the session-list equivalent: let a provider still be fully usable (via its own panel), while excluding its sessions from the shared/merged session list.

## Proposed Solution

Add a setting (or per-provider context menu action in the sessions list) to exclude a specific agent/provider's sessions from the merged Chat view / Agent Sessions list, without disabling the provider or preventing it from being used elsewhere (e.g. its own dedicated panel).

Example:
```jsonc
"chat.agentSessions.excludedProviders": ["openai.codex"]
```

## Related Issues
- #318836 (chat.agentHost.enabled doesn't hide remote sessions)
- #277696 (hidden agents become unusable)
- #282607 (Hide Agent Sessions Sidebar doesn't work)