[Bug]: Worktree threads always write a full checkout, which fills the disk on large monorepos #26851

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Add a large monorepo as a project. The example here is a Unity game repository with a working tree of about 30 GB and 277k files.
2. Start three threads in a new worktree, or run three delegate_task calls with worktree isolation.
3. Watch the free space on the drive that holds the worktrees directory. The default is ~/.t3/worktrees, on the system drive.

Expected behavior

A large monorepo often keeps its working tree small with git sparse checkout. T3 worktrees can use the same sparse-checkout patterns, so a new worktree writes only the files that an agent needs.

Actual behavior

Every T3 worktree runs a full git worktree add in GitVcsDriverCore.createWorktree, which writes every tracked file. In the example repository, each worktree wrote about 30 GB. Three worktree threads used about 90 GB, and the 1 TB system drive ran out of space.

A runOnWorktreeCreate project script cannot prevent this, because it runs after git has written every file. With the repository's non-cone sparse-checkout pattern file, the same worktree checks out as 1.79 GB and 63k files in about 36 s.

The problem does not depend on Unity. Any repository with a large working tree gets one full copy per worktree thread: a game repository with binary assets, a monorepo with many packages, or a repository with vendored dependencies.

Impact

Major degradation or frequent failure

Version or commit

main @ fe219c4

Environment

Windows 11 Pro x64, Git 2.45.2.windows.1, T3 Code desktop

Workaround

Move the worktrees directory to a larger drive, or create the worktree outside T3 with git worktree add --no-checkout, git sparse-checkout set --no-cone --stdin, and git checkout. The second route does not cover delegate_task worktree isolation or PR threads, which always go through createWorktree.

Proposal and fix

• Proposal: discussion 26819, an optional worktreeSparsePatternsPath field in t3.json.
• Fix: PR 26820.
• Related large-monorepo issues on the checkpoint side: issue 3646, issue 12924, and issue 13741. Checkpoint capture already supports sparse checkouts. Worktree creation does not.

Done with Claude Opus 5.5 in Claude Code, run from T3 Code, with Claude Fable 5.1 and GPT-6 Astra looped in.