feat(board): export board data as JSON

[afurm]

Local-first users need a simple way to back up or inspect their board data without copying SQLite files manually.

Suggested scope:

• Add an export path that returns board tasks as JSON.
• Include task metadata, status, priority, focus area, tags, descriptions, and execution summaries where useful.
• Exclude secrets: API keys, OAuth tokens, provider credentials, and private memory file contents.
• Add a small Settings or board action that downloads the export.

Acceptance criteria:

• Users can export their board from the app without OpenAI credentials.
• The export format is deterministic enough to test.
• Server tests verify secrets are not included.
• npm run typecheck, npm test, and npm run build pass.