Isolate Buzz-managed Codex sessions from the user's Codex sidebar and Remote project list

## Summary

Buzz Desktop launches its managed Codex agents from `$HOME/.buzz`, but the child Codex processes use the user's default `$HOME/.codex` state store. This causes `.buzz` and its managed-agent sessions to appear in the user's personal Codex project/history UI, including the ChatGPT iOS **Remote** project list.

Users who interact with these agents exclusively through Buzz should not have Buzz's internal workspace mixed into their personal Codex sidebar.

## Steps to reproduce

1. Install and launch Buzz Desktop.
2. Use Codex as the preferred local agent runtime and start one or more managed agents.
3. Open Codex Desktop and/or ChatGPT on iOS → **Remote**.
4. Observe `$HOME/.buzz` listed as a Codex project, with Buzz-managed agent sessions associated with it.

## Observed behavior

On the reproduced macOS installation:

- Buzz agents run with `cwd=$HOME/.buzz`.
- The child `codex` processes open `$HOME/.codex/logs_2.sqlite` and rollout files under `$HOME/.codex/sessions/...`.
- `$HOME/.buzz/.codex` exists but is not being used as the Codex state root.
- The normal Codex state database contains unarchived ACP-created threads whose `cwd` is `$HOME/.buzz`.
- Consequently, `.buzz` is exposed as a project in the user's Codex/Remote UI.

## Expected behavior

Buzz-managed agent sessions should remain managed by and visible through Buzz unless the user explicitly opts into exposing them to personal Codex history.

## Suggested direction

Either:

1. Launch Buzz's Codex adapter with a Buzz-owned `CODEX_HOME` such as `$HOME/.buzz/.codex`, including an appropriate authentication/config handoff; or
2. Start Codex app-server threads with `ephemeral: true` when Buzz owns the durable conversation state and Codex-side persistence is unnecessary.

An explicit **Expose managed sessions in Codex history** option could be provided if some users want the current behavior, preferably defaulting off.

## Environment

- macOS
- Buzz Desktop `0.4.23`
- `@agentclientprotocol/codex-acp` `1.1.7`
- Codex selected as the preferred runtime
- ChatGPT iOS Remote view

## Related

- #2334 tracks the separate problem of managed ACP sessions receiving unreadable/duplicated titles in Codex. Isolating managed sessions addresses visibility; title metadata would still matter when exposure is intentionally enabled.

Start at the Codex adapter launch path and inspect how Buzz-managed child codex processes receive state configuration, comparing the $HOME/.buzz and $HOME/.codex paths described here. Done means managed sessions no longer appear in the user's Codex or Remote history by default, with any chosen authentication handoff or opt-in exposure behavior working as specified.

Written by the indexing model from the issue text.