feat: ship Assistant safety, subagent orchestration, and onboarding

[sambitcreate]

Release scope

• Adds a first-run onboarding flow for selecting and configuring an AI provider, with regression coverage for display state and provider defaults.
• Hardens scheduled Assistant automation with exact provider and MCP bindings, safe approval contracts, bounded tool recovery, and schedule persistence guards.
• Ships Subagent V2: capability narrowing, exact approval-bound effects, durable foreground/background lifecycle, nested orchestration, MCP/web/shell/workspace boundaries, and accessible renderer controls.
• Adds native helpers and adversarial regression coverage for safe workspace mutation and shell execution.
• Retains MCP deadlines through configuration, inventory, read, and mutation paths; updates the MCP SDK to its security release.

Validation

• npm test — 1,184 application tests plus native worktree and Computer Use suites
• npm run type-check
• npm run lint
• npm run test:branding — 24 tests
• npm run test:native — 4 tests
• npm run build
• git diff --check

Hosted CI

GitHub Actions is queued on the macOS 26 runner pool. The integration-only rerun was independently locally validated before this draft was opened; this release PR is intentionally not being merged to main until its hosted checks report.