isolation: "worktree" silently fails — agent runs in main repo instead of isolated worktree

Description

When spawning a subagent with isolation: "worktree", the agent silently runs in the main repository directory instead of an isolated worktree. No error is raised — the worktree simply never gets created.

Steps to Reproduce

1. Open a project with a git repository
2. Spawn a subagent with isolation: "worktree":

``
Agent({
description: "Test worktree isolation",
isolation: "worktree",
prompt: "Run pwd and git branch --show-current"
})
``

1. Observe the agent's output

Expected Behavior

• Agent should run in a temporary git worktree (e.g., /tmp/worktree-<hash>)
• Agent should be on its own branch, isolated from the main checkout
• pwd should show the worktree path, not the main repo
• worktreePath in the result should be an actual path
• worktreeBranch should be defined

Actual Behavior

• Agent runs in the main repository directory (same pwd as parent)
• worktreePath: done (not an actual path)
• worktreeBranch: undefined
• Agent commits directly to the main checkout's branch
• Zero isolation — the agent modifies shared git state

Impact

This makes it impossible to run parallel agents that work on different branches. When multiple agents share the same .git state:

• Branch checkout races: Agent A checks out branch X, Agent B checks out branch Y → Agent A is now on branch Y
• Commits land on wrong branches
• Changes "leak" to unintended branches (including main)

We lost a full morning diagnosing this before discovering the root cause.

Workaround

• Use parallel agents only for read-only tasks (search, read, analyze)
• Perform all git mutations (checkout, commit, push) sequentially from the main conversation thread
• Never dispatch two agents that both need different branches

Environment

• Claude Code CLI (Opus 4.6, 1M context)
• macOS Darwin 24.3.0
• Git 2.x
• Repository with standard .git structure

Diagnostic Evidence

Agent dispatched with: isolation: "worktree"
Result metadata:
  worktreePath: done
  worktreeBranch: undefined

Agent reported:
  pwd: /Users/.../stablebridge-tx-recovery  (MAIN REPO, not a worktree)
  git branch: test/subagent-diagnostics     (MAIN REPO's branch)
  git commit: succeeded                      (committed to MAIN REPO)
  git worktree list: only 1 worktree (main) (no additional worktree created)