bug: VSCode sequential worker limit blocks other targets from running concurrently

[christso]

Problem

When an eval specifies multiple targets (e.g., vscode + copilot), the VSCode provider's sequential worker limit (workers=1) prevents other targets from running concurrently. The warning message is:

Warning: VSCode providers require window focus. Limiting workers from 3 to 1 to prevent race conditions.

This limit should only apply to VSCode provider instances, not to other providers like copilot that can run independently.

Expected Behavior

• VSCode evals should be serialized (1 worker for VSCode)
• Other targets (copilot, claude, etc.) should run concurrently alongside VSCode
• Each target type should have its own concurrency control

Actual Behavior

The worker limit is applied globally across all targets, so copilot waits for VSCode to finish (or vice versa).

Reproduction

execution:
  targets:
    - vscode
    - copilot

tests:
  - id: verify-multi-repo
    input: Read package.json
    assert:
      - type: contains
        value: agentv

Only VSCode runs; copilot appears to be blocked.

Context

Found during #458 — adding multi-target workspace example.