fix: Gate creative fate storyboard on library support

[mvanhorn]

Summary

Add the existing requires_capability predicate for creative.has_creative_library == true at storyboard level in creative_fate_after_cancellation.yaml, and increment the storyboard's patch version because its applicability contract changes. Extend the existing SDK-backed capability-gate regression suite to load the real storyboard and prove that an explicit false declaration skips the whole storyboard even when all required tools are auto-registered, while a true declaration passes the gate; strip executable phases only for the affirmative check so the test exercises selection without network calls. Add a descriptive protocol patch changeset explaining that library-less sellers now exclude this storyboard from runnable coverage and therefore may see corrected badge/completeness denominators.

Validation

• With creative.has_creative_library: false and every storyboard tool present in agentTools, running the real storyboard returns one whole-storyboard capability_unsupported/not_applicable skip, performs no scenario steps, and records no failure.
• With creative.has_creative_library: true, the same storyboard satisfies the gate; a non-executable copy with prerequisites and phases removed reaches the normal no_phases result instead of capability rejection.
• With the capability path absent, the storyboard remains not applicable rather than inferring support from sync_creatives or list_creatives tool registration.
• The compliance source build/check accepts the gated storyboard and the changeset communicates the expected runnable-scenario and badge/completeness-score shift without modifying a released semver artifact.

Why

media_buy_seller/creative_fate_after_cancellation calls sync_creatives and list_creatives but has no storyboard-level applicability gate, so a seller explicitly declaring creative.has_creative_library: false can be selected and then fail outside its advertised scope. The repository contract says an unsatisfied storyboard-level requires_capability gate must produce a clean not_applicable result before tool and phase execution. This plan covers the independently mergeable patch for that storyboard only; it does not introduce the compound gate needed by the other three scenarios or rewrite immutable 3.0/3.1 release artifacts.

Closes #6701

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