chore: drop open-source licensing and submission feature

The project is no longer open-source. Migrate from AGPL-3.0-only to
UNLICENSED (npm's proprietary marker) and remove the workflow
open-source submission feature end to end.

License
- AGPL-3.0-only -> UNLICENSED in package.json, backend/package.json,
  and frontend/package.json.
- Delete LICENSE.
- Lockfiles regenerated to reflect the license field change.

Database
- New idempotent migration
  backend/migrations/20260727_01_drop_workflow_open_source_submissions.sql
  drops the workflow_open_source_submissions table for existing
  databases.
- Delete the now-redundant 20260629_01_workflow_open_source_submissions.sql
  migration.

Backend
- Remove POST /workflows/:workflowId/open-source from
  backend/src/routes/workflows.ts.
- Remove OpenSourceSubmissionStatus, OpenSourceSubmissionRow, and
  OpenSourceSubmissionSummary types from workflows.ts.
- Remove withOpenSourceSubmission, toOpenSourceSubmissionSummary,
  getLatestOpenSourceSubmission, buildOpenSourceSnapshot, and
  validateOpenSourceWorkflow helpers from workflows.ts; stop injecting
  the open_source_submission field on workflow GET responses.
- Drop the workflow_open_source_submissions reads from
  backend/src/lib/userDataCleanup.ts and backend/src/lib/userDataExport.ts.
- Update stack.supabase.test.ts and userDataCleanup.test.ts fixtures.

Frontend
- Delete frontend/src/app/components/workflows/OpenSourceWorkflowModal.tsx.
- Remove WorkflowOpenSourceSubmission, OpenSourceWorkflowResponse,
  OpenSourceWorkflowContributorMode, and WorkflowContributor types and
  the open_source_submission field from
  frontend/src/app/components/shared/types.ts.
- Remove openSourceWorkflow() and the orphan WorkflowContributor import
  from frontend/src/app/lib/courierApi.ts.
- In WorkflowDetailPage.tsx: drop the OpenSourceWorkflowModal + Globe
  icon imports, the WORKFLOW_CONTRIBUTIONS_ENABLED env constant, the
  canOpenSource const, the openSourceOpen / openSourcePending /
  defaultContributorName locals, the open-source action item push,
  the open_source_submission spreads in saveColumns and onUpdated, and
  the <OpenSourceWorkflowModal/> JSX.
- In NewWorkflowModal.tsx: scrub open-source copy.
- In CitationSources.tsx: drop the onOpenSource / canOpenSource props
  and their internal usage.
- In AssistantMessage.tsx: drop the onOpenSource / canOpenSource props
  on the CitationsBlock call site and the dead
  handleOpenCitationSource / canOpenCitationSource helpers.

Docs / CI
- Scrub 'open-source' phrasing from docs/safe-local-testing.md,
  docs/e2e-ci.md, and the matching comment in
  .github/workflows/e2e.yml.

Validation
- backend tsc, frontend tsc, and the 259-test backend vitest suite
  pass; both backend and frontend npm installs refresh the lockfiles
  cleanly with only the projects' own license entry changing.