fix(dashboard): drop unfiltered /api/storyboards fallback (closes #4254)

[bokelley]

Summary

Closes #4254. The dashboard's storyboard picker was falling back to the unfiltered full catalog when applicable-storyboards returned a generic error, defeating the per-agent filter.

The bug

The dashboard renders the storyboard picker by calling /api/registry/agents/:url/applicable-storyboards, which uses resolveStoryboardsForCapabilities to filter to (universal baseline + protocol baseline + declared specialisms). That endpoint works correctly.

But when it returned anything other than 2xx / OAuth-challenge / needs-auth / unknown-specialism (e.g. capabilities probe failed for a different reason), the dashboard fell back to /api/storyboards — the unfiltered, registry-wide catalog. So a signals-only agent rendered pagination-creative-formats, get-media-buys-pagination-integrity, collection-lists-pagination-integrity, and the rest. The issue report's "20+ storyboards including pagination-integrity (list_creatives)" is exactly this code path firing.

Fix

Drop the fallback. Keep the error message; suffix it with the underlying contract pointer (get_adcp_capabilities must return supported_protocols and specialisms before applicable storyboards can be computed). OAuth, needs-auth, and unknown-specialism error paths unchanged — those still surface their respective actionable UIs (authorize button / connect form / specialism list).

Test plan

• Owner dashboard on an agent whose capabilities probe succeeds → renders filtered applicable list (current behavior, unchanged)
• Owner dashboard on an agent that requires OAuth → renders Authorize button (unchanged)
• Owner dashboard on an agent that requires saved auth → renders connect form (unchanged)
• Owner dashboard on an agent that declared an unknown specialism → renders specialism error + known-specialisms list (unchanged)
• Owner dashboard on an agent whose capabilities probe failed for a generic reason → renders the error message without falling back to the full catalog

Stack note

Part of Push A in the compliance reporting fidelity initiative (PR #4364 → this PR → docs-disambiguation PR → canonical-surface UI). Orthogonal to Emma's #4247 unification stack.

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