fix(dashboard): remove unfiltered storyboard fallback on probe failure

[bokelley]

Closes #4254

When the capability probe fails for a registered agent, the Test view's renderStoryboardPicker() was falling back to GET /api/storyboards — the full unfiltered catalog — and rendering 20+ storyboards regardless of the agent's declared capabilities. A signals-only agent would see get-media-buys-pagination-integrity, list_creatives pagination storyboards, and other media-buy tests it would never be required to pass. The "Showing all available storyboards:" subheading was not sufficient to prevent confusion; members were clicking Run against a still-unreachable agent and getting a second error.

Non-breaking justification: this is a UI-only change inside server/public/dashboard-agents.html; it removes a misleading fallback fetch and replaces it with structured error messages. No API surface, schema, or protocol contract is changed.

What changed:

1. Removed the /api/storyboards fallback — when the probe fails with a non-classified 500/504, no storyboard list is shown. The member needs to fix their agent first.

2. Added specialism_parent_protocol_missing branch — was previously falling through to the generic error path (and then the fallback). Now shows "Capabilities misconfigured. Specialism X requires Y in supported_protocols. Add Y to your get_adcp_capabilities response."

3. Added per-reason guidance using the existing reason enum the server already returns (network | tls | timeout | protocol | unknown) — gives the member a one-sentence actionable hint matched to the failure mode instead of a raw (network) tag appended to the error string.

4. Mapped HTTP 504 explicitly to the timeout hint — the server returns { error: "Connection timeout" } with no reason field on 504, so res.status === 504 check ensures the right guidance rather than falling to the generic fallback.

Pre-PR review:

• code-reviewer: approved — no blockers; XSS handling correct throughout; nit: add explicit unknown key to reasonHints map for self-documentation
• internal-tools-strategist: approved — right UX decision; nits: (1) move specialism_parent_protocol_missing check before unknown_specialism for logical priority order; (2) tls hint copy is descriptive rather than imperative ("Renew or replace..." fits the register better); (3) fallback string in specialism_parent_protocol_missing branch re-uses errBody.error which includes "misconfigured" twice — use specialism/parent_protocol fields directly

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See #3121
for context.

Session: https://claude.ai/code/session_011RjJrMxXuaEQXgg6HJMWqA

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