Consolidate test utilities into single location

[dcramer]

Test utilities are split between cli/test-helpers.ts and test-utils/ directory, making it unclear where to add new test utilities or find existing ones.

Exploration needed:

1. Audit contents of cli/test-helpers.ts - what does it provide?
2. Audit contents of test-utils/ - what does it provide?
3. Check if mcp/test-helpers.ts also exists
4. Determine if separation is intentional (CLI-specific helpers vs general)
5. Review how tests currently import these utilities
6. Check for any duplicate or overlapping functionality

Options:
a) Move everything to test-utils/ with subdirectories if needed
b) Keep separation but document clearly when to use which
c) Create test-utils/index.ts that re-exports everything

Goal: Clear convention for where test utilities live and how to import them.