refactor(create-expert): document why exec is used in expert-tester

[FL4TLiN3]

Summary

Add documentation explaining why exec is used in expert-tester instead of delegation.

Changes

Added explanation note in expert-tester instruction:

NOTE: We use `exec` instead of delegation because we need to test the Expert as a black-box,
exactly as end-users would run it via the CLI. This ensures realistic test conditions.

Rationale

Per user decision: documentation only, no design changes. The expert-tester uses exec to invoke perstack CLI commands rather than delegating to the expert under test. This is intentional because:

1. Black-box testing: Tests should exercise the Expert exactly as users would
2. Realistic conditions: CLI execution includes all real runtime behaviors
3. End-to-end verification: Catches issues that might not appear with delegation

Closes #357

🤖 Generated with Claude Code

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Note

Documentation-only update

• In create-expert-toml.ts, updates expert-tester instructions to explain using exec (instead of delegation) to run experts as a black-box via CLI for realistic test conditions
• Adds .changeset/document-exec-usage.md to record the patch-level change for create-expert

No functional code changes

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✅ All modified and coverable lines are covered by tests.

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