fix(compliance): replace hardcoded capability_id fixture value with field_present check

[bokelley]

Summary

Fixes canonical_supported_formats storyboard: the capability_discovery phase incorrectly asserted a fixture-specific capability_id value ("training_image_generation") as a required protocol validation, causing every creative agent with a different — but equally valid — capability identifier to fail compliance.

• Changed check: field_value (hardcoded "training_image_generation") to check: field_present on creative.supported_formats[0].capability_id, so the check validates structure rather than a fixture constant.
• Removed a companion field_absent check on creative.supported_formats[1].capability_id whose own description identified it as a fixture constraint ("only advertises implemented canonical build paths"), not a protocol requirement.

creative.supported_formats[].capability_id is an agent-local stable identifier — a free-form string each creative agent defines independently. The protocol schema (get-adcp-capabilities-response.json) defines it with no enum constraint, and the storyboard narrative itself states it is "agent-local". The data flow from context_outputs to $context.image_build_capability_id in the build phase is unaffected.

Closes #5843

Test plan

• canonical_supported_formats storyboard passes for a creative agent with any capability_id value
• canonical_supported_formats storyboard still correctly validates that a capability_id is present (structure check intact)
• canonical_supported_formats storyboard still validates format.format_kind: "image" (protocol-level assertion unchanged)
• Context output image_build_capability_id is captured and consumed by the subsequent build phase
• Changeset scope check passes (patch for adcontextprotocol)

---

Generated by Claude Code