feat(workspace): add context field for AI agent background info

Add a `context` text field to workspaces, allowing users to provide
background information and context for AI agents working in the
workspace. Full stack: migration, sqlc queries, Go handler, TS types,
SDK, and settings page UI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/test/helpers.tsx

@@ -19,6 +19,7 @@ export const mockWorkspace: Workspace = {
   name: "Test Workspace",
   slug: "test-ws",
   description: "A test workspace",
+  context: null,
   settings: {},
   created_at: "2026-01-01T00:00:00Z",
   updated_at: "2026-01-01T00:00:00Z",