[Wishlist] Pin Agent environment per project/thread instead of a global Windows/WSL toggle

Feature request

Allow Codex Desktop on Windows to select and persist Agent environment per project and per thread, instead of treating Windows vs WSL as a global toggle that retroactively changes how existing threads are resumed.

Motivation

A Windows workstation commonly contains both kinds of projects:

• native Windows repositories that should use the Windows agent/sandbox; and
• Linux repositories under /home/... that should use the WSL agent, Linux filesystem semantics, and Linux tooling.

The current global toggle makes this mixed workflow unsafe. Switching from WSL to Windows can preserve a thread ID while changing the execution host and path dialect underneath it. Existing WSL rollouts can then fail to resume in the Windows backend with errors such as:

AbsolutePathBuf deserialized without a base path

It can also create invalid projections such as C:\home\<wsl-user> for a canonical WSL project.

Related evidence:

• #28094 — project/thread identity and cross-backend path/resume failures
• #37104 — WSL Agent integrated-terminal failures, while Windows Agent + WSL terminal can work

Requested behavior

1. Project default: each project stores agentEnvironment = windows | wsl:<distro>.
2. Thread pinning: a thread stores the environment, distro/host identity, and canonical cwd used when it was created.
3. No retroactive mutation: changing the global/default environment affects only new projects or new threads.
4. Explicit migration: reopening a thread under another environment requires an explicit fork/migrate action with a preview of cwd/path conversion.
5. Visible identity: show a Windows or WSL badge plus canonical cwd in the thread header/sidebar.
6. Mixed concurrent use: Windows projects may run with the Windows agent while WSL projects run with the WSL agent in the same Desktop session.
7. Safe terminal default: the integrated terminal should inherit the thread's pinned environment unless explicitly overridden.
8. Typed-path migration: if migration is supported, convert all persisted typed path fields and rollout metadata using a versioned migration rather than parsing them under a different host dialect.

Example

• C:\src\windows-app → Windows agent
• \\wsl$\Ubuntu\home\user\linux-app / /home/user/linux-app → WSL Ubuntu agent

Both projects should remain usable simultaneously, and switching the default for future projects should not change either existing thread.