Workspace providers: pluggable create/remove commands (rift, CoW clones, Worktrunk), with support for non-git directories and cleanup on settle

[evbuildsnet]

Problem

T3 Code has one way to isolate a thread: git worktree add into ~/.t3/worktrees. That path is hardcoded in GitVcsDriverCore.createWorktree, and everything around it assumes git:

• A project that is not a git repository can never get an isolated workspace. It silently falls back to local, so every thread on that project shares one directory.
• A fresh worktree is a cold checkout. node_modules, .env, build caches and generated files are gone, and the runOnWorktreeCreate action has to rebuild them for every thread.
• The workspace is never released. Settling a thread leaves the directory on disk ([Feature]: Clean up worktrees for settled threads without deleting history #6856), and removal only happens on thread delete, as a client-side checkbox.

Several discussions already ask for a piece of this: copy-on-write clones instead of worktrees (#10853), driving worktrees through Worktrunk (#8953), and cleaning up when a thread settles (#6856, #6999, #6864). They are all the same missing abstraction.

Proposal: workspace providers

Make the git worktree a provider, and let a project pick a different one. A provider is three things, configured in t3.json or project settings:

{
  "workspace": {
    "create": "rift create --copy-all",   // runs in the project dir, prints the workspace path
    "remove": "rift remove --force",      // runs inside the workspace
    "requiresGit": false                  // default true; false lets plain directories use it
  }
}

Lifecycle:

1. New thread → run create in the project directory, take the printed path as the thread's worktreePath, start the session there. Existing runOnWorktreeCreate actions keep working since they only need the path.
2. Thread settled / archived → run remove inside the workspace. Same guards the team named when closing feat(server): clean worktree build artifacts when threads settle #8771: skip while a session is running, while terminals are open, or while another thread references the path.
3. Thread unsettled → run create again, exactly like the current "recreate a missing worktree on the next turn" behaviour.

The built-in git provider is git worktree add / git worktree remove with requiresGit: true, so nothing changes for anyone who does not opt in.

Motivating example: rift

rift makes copy-on-write clones of a directory using APFS clonefile. It does not care whether the directory is a git repo, or whether it holds several repos side by side. rift create --copy-all gives a full clone in well under a second, including node_modules and build output, so the agent starts warm. rift remove moves it to trash and unregisters it. It already has a .rift.toml postcreate hook and exposes RIFT_SOURCE / RIFT_DESTINATION to it.

I run this today by hand for every thread. The only thing missing is T3 Code calling it, and pointing the thread at the result.

Worktrunk (#8953) fits the same contract: wt switch --create --json <branch> prints a path, wt remove tears it down. A plain cp -c CoW clone (#10853) fits too.

Smallest useful scope

• workspace.create and workspace.remove in t3.json, requiresGit flag.
• Server runs create where git worktree add runs today and stores the path on the thread.
• A reactor on thread.settled that runs remove with the guards above.
• Non-git projects are allowed to use worktree mode when the provider does not require git.

No UI beyond what project settings already have for actions. Happy to contribute the implementation if this shape is welcome, and to fold it into the post-Orchestrator-V2 worktree lifecycle work the team mentioned in #8771.