fix: use examples for business letter starter

[quick-ricon]

Summary

• replace the new emails template surface with emails example
• move business-letter.tsx into the existing examples/ catalog
• update README and source comments to use emails example business-letter > draft.tsx
• rename the BATS suite from template to example and keep business-letter compose coverage

Context

Fix-it for #39 after Or noticed that emails already has an examples collection. This keeps one starter/example surface instead of splitting examples/ from a new templates/ directory.

Validation

• mise run test example send — 33/33
• mise run test — 103/103
• mise run test-integration — 35/35
• bun test src/ — 44/44
• mise exec -- readme build --check
• codebase lint "$PWD" — 9/9 in a correctly named emails worktree
• git diff --check origin/c0da/business-letter-template...HEAD
• residual scan for stale emails template / templates/business-letter / Business letter template references