Migrate board CLI into src/cli directory

[artiphishle]

What changed

• moved the standalone CLI entrypoint from src/cli.ts to src/cli/index.ts
• updated internal imports, README usage, and CLI tests
• changed the published binary target to ./dist/cli/index.js
• declared the library, provider, and CLI source entrypoints explicitly for Knip
• updated the package metadata assertion
• added a patch changeset

Why

ankh doctor validate . rejects the top-level src/cli.ts layout. The CLI now follows the expected src/cli/ directory structure while preserving its existing behavior.

Validation

GitHub Actions CI passes:

• build
• typecheck
• lint
• format check
• Knip
• tests
• docs generation
• changeset check