(CODUX-216) Wire agent attachments into the canonical Nodes workspace - #2976

## Objective
Complete the dashboard agent attachment integration required by T17 so the canonical project-scoped Nodes workspace can manage governed flow attachments.

## Scope
- `dashboard/src/v2/NodesPage.tsx`
- `dashboard/src/v2/lib/agent-preset-api.ts`
- `dashboard/src/v2/lib/node-flow-api.ts`
- `dashboard/src/v2/components/nodes/NodeFlowInspector.tsx` only if required for integration
- Dashboard tests under `tests/dashboard/v2/` and related dashboard library tests

## Requirements
1. When a project is selected, load its agent presets through the existing project-agent API. When a flow is selected, load that flow's attachments through the existing node-flow attachment API.
2. Maintain attachment state, selected agent state, loading state, mutation state, and user-visible error handling. Abort or ignore stale requests when the selected project or flow changes.
3. Wire attach to `attachNodeFlowToAgent` and detach to `detachNodeFlowFromAgent`. Refresh the selected flow's attachments after each successful mutation, clear or preserve selection deterministically, and keep project ownership enforced by the backend APIs.
4. Pass the actual agents, attachments, selected agent id, mutation state, and attach/detach callbacks into `NodeFlowInspector`; remove the hard-coded empty arrays and no-op callbacks currently used by `NodesPage`.
5. Preserve metadata-only behavior: attachment UI may show agent and skill metadata but must never render credentials, decrypted values, or custom source.
6. Preserve accessible loading, error, empty, project-switching, and keyboard interaction states.

## Verification
- Add tests covering agent preset loading, existing attachment loading, attach, detach, project switching, failed API calls, loading/error recovery, and keyboard accessibility.
- Run `pnpm run typecheck:dashboard`.
- Run focused Nodes dashboard tests and the full dashboard suite, using single-worker execution if required by the validation environment.
- Run `pnpm run build` and `pnpm run test:e2e:credentialed-automation`.

## Constraints
- Reuse the existing dashboard fetch helpers and governed backend contracts.
- Do not add browser-local persistence for attachments.
- Do not weaken project ownership or attached-flow capability authorization.