PlateSpinner: Deep-link toast click-through to subworkflow node context

[PolyphonyRequiem]

Context (revised 2026-05-28)

Domain signals carry \cta_url\ + \cta_kind\ for click-through. When a toast fires for an \�ction_required\ signal emitted deep inside a nested subworkflow (e.g., \polyphony → root-batch-dispatch → root-item-dispatch → implement-merge-group → github-pr), clicking the toast should navigate to the subworkflow context where the signal was emitted — not just the top-level run.

The Gap

PlateSpinner toasts currently link to /?run={run_id}. The CTA-based rendering (#541) will open \cta_url\ in the browser (the PR itself), but the dashboard card / bell entry should deep-link into the subworkflow for the operator who wants workflow context rather than just the external artifact.

PlateSpinner already supports ?subworkflow=...\ for breadcrumb navigation. Gaps:

1. Dashboard cards and bell entries don't carry subworkflow path context
2. No &node=...\ parameter for highlighting the specific signal entry in the activity log
3. No auto-scroll-to-signal on deep-link arrival

Acceptance Criteria

• Dashboard card for a domain signal includes a secondary 'View in workflow' link → navigates to the subworkflow context
• Activity log auto-scrolls to / highlights the signal entry on deep-link arrival
• Falls back to run-level if subworkflow path unavailable

Relationship

Part of Epic #521. Depends on #541 (CTA-aware rendering) shipping first. This is P2 polish — the primary user flow (click CTA → open PR in browser) works without this.

Design Reference

.squad/decisions/inbox/bach-on-error-notifications-design-2026-05-28T155810-0700.md\ §2.2