fix(roam): format imported bullets with tab indents

[markoub]

Summary

• Format Roam JSON child blocks so first-level bullets start at the beginning of the line.
• Use tab indentation for nested Roam bullets, matching lists created in Obsidian.
• Add focused coverage for Roam bullet line formatting.

Test

• pnpm install --frozen-lockfile
• pnpm exec tsx --test tests/roam/block-refs.test.ts tests/roam/bullet-formatting.test.ts
• pnpm exec eslint src/formats/roam-json.ts src/formats/roam/list-format.ts tests/roam/bullet-formatting.test.ts
• pnpm run test
• pnpm run build

Fixes #250