feat: add Roam Research integration

[salmonumbrella]

Summary

• Add Roam Research as a notes integration, following the existing Obsidian pattern
• Uses the Roam Desktop local API shape from current roam-tools for page creation, block creation, search, and page reads
• Saves plan contents inside a fenced markdown block in Roam, while keeping parsed frontmatter as searchable Roam attributes outside the block
• Includes settings UI, reference browser, export/default-save support, and parity coverage for both Bun and Pi server runtimes

Test Plan

• bun test packages/server/roam-client.test.ts packages/server/roam-reference.test.ts packages/server/integrations.test.ts packages/ui/utils/roam.test.ts packages/ui/utils/defaultNotesApp.test.ts packages/ui/components/ExportModal.test.tsx packages/ui/components/sidebar/FileBrowser.test.tsx apps/pi-extension/server/roam-client.test.ts
• bun run typecheck
• bun test (1414 tests)
• bun run --cwd apps/review build && bun run build:hook && bun run build:opencode