feat(core): expand and make configurable default exclude patterns

[aridyckovsky]

Expand default exclusion patterns in config to cover common build artifacts and make them easily configurable.

Proposed Additions:

exclude: [
  'node_modules/**',
  'dist/**',
  'build/**',
  'coverage/**',
  '.next/**',
  'out/**',
  '.turbo/**',
  '.cache/**'
]

Tasks:

1. Update default config template
2. Document override/merge behavior
3. Consider allowing exclusions at multiple levels (global + per-pattern)
4. Add tests for exclusion scenarios

File: effect-migrate.config.ts

Review: docs/agents/prs/reviews/amp/feat-thread-command.md