fix(agent-manager): speed up worktree session startup

[marius-kilocode]

What Problem This Solves

Creating an Agent Manager worktree waits on Git checkout and then keeps the successful setup overlay visible after the session is ready. Large repositories also spend unnecessary time enumerating remote-tracking branches during local branch collision checks.

Why This Change Was Made

Use Git's built-in parallel checkout with two workers when the repository has no explicit checkout.workers setting, while preserving explicit user configuration. Limit collision detection to local branch refs, which are the only refs that can conflict with a new local branch. Dismiss successful setup feedback immediately after readiness while retaining the error delay.

User Impact

The first usable Agent Manager session was measured at approximately 4.37 seconds before these changes and 2.65 seconds after them.

Measurement	Before	After	Improvement
Complete startup to usable session	4.37 s	2.65 s	1.72 s, 39%
Git checkout	3.15 s	2.12 s	1.03 s, 33%
Local branch collision lookup	218 ms	25 ms	193 ms, 89%
Successful setup overlay delay	500 ms	Immediate	500 ms

The measurements used a disposable repository with 9,640 tracked files and 424 Git LFS files. The total varies with repository size, Git LFS content, disk speed, hooks, setup scripts, remote latency, and host load.

Safety Validation

The checkout still uses a fresh remote commit, standard git worktree add, Git LFS, and post-checkout hooks. Explicit user-defined checkout.workers values remain unchanged. Branch collision tests cover local branches and remote-tracking refs.

• Focused Agent Manager tests passed, including explicit worker configuration, branch collisions, post-checkout hook failure tolerance, and setup overlay timing.
• Extension typecheck, lint, compile, formatting, and check-kilocode-change passed.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (5 files)

• .changeset/faster-agent-manager-worktree-start.md
• packages/kilo-vscode/src/agent-manager/WorktreeManager.ts
• packages/kilo-vscode/tests/unit/agent-manager-arch.test.ts
• packages/kilo-vscode/tests/unit/worktree-manager.test.ts
• packages/kilo-vscode/webview-ui/agent-manager/AgentManagerApp.tsx

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_{Review guidance: REVIEW.md from base branch main}