Problem

Closes #177.

Investigators can copy selected node labels as text today, but there is no built-in way to export the selected graph subset as a machine-readable artifact for reports, backups, or handoff to other tools.

Before / after

Before:

• The selected-node context menu only exposed the existing text copy action.
• Exporting a selected graph subset required manual work outside Flowsint.

After:

• The selected-node context menu includes an Export ... as JSON action.
• The downloaded JSON includes:
  • exported_at
  • the selected nodes
  • edges whose source and target are both in the selected node set

Verification

• corepack yarn install --frozen-lockfile
• corepack yarn prettier --check flowsint-app/src/components/sketches/graph/context-menu/background-context-menu.tsx
• git diff --check

Additional checks attempted:

• corepack yarn workspace flowsint-app typecheck is still blocked by existing project-wide TypeScript errors unrelated to this file, including missing Tiptap modules and table/router typing errors.
• A file-level ESLint run is still blocked because .eslintrc.cjs requires @typescript-eslint/parser, but that parser is not installed by the lockfile.