feat(compliance): add signals-specific universal storyboards for error_handling and core tracks

[bokelley]

Refs #3350

Summary

Signals-only agents (supported_protocols: ["signals"], no get_products tool) get 0 steps from the error_handling and core compliance tracks in the 5.13 runner. Root cause: error-compliance.yaml and schema-validation.yaml both carry required_tools: [get_products] at the storyboard root — the runner interprets this as an applicability gate and emits missing_tool / skips the storyboard entirely for agents that lack get_products.

This PR adds two new universal storyboards targeting signals agents:

• error-compliance-signals.yaml (track: error_handling, required_tools: [get_signals, activate_signal]) — 5 phases: capability discovery, error responses (nonexistent segment ID, missing required field), error structure validation, VERSION_UNSUPPORTED / version negotiation, MCP transport binding
• schema-validation-signals.yaml (track: core, required_tools: [get_signals]) — 2 phases: capability discovery, schema compliance (required fields on signals, pricing options structure)

Updates docs/building/conformance.mdx and docs/building/compliance-catalog.mdx per the doc-parity lint requirement.

Non-breaking justification: adds new storyboards; no existing storyboard is modified; media-buy agents are unaffected. Signals agents gain new required compliance coverage — this was always the spec intent (universal tracks must run for all agents), so this restores correct behavior rather than expanding requirements.

Note: The existing error-compliance.yaml and schema-validation.yaml still have required_tools: [get_products] at the root, which means they remain scoped to media-buy agents. A complementary fix in the adcp-client runner (using supported_protocols for track applicability rather than required_tools as a proxy) would be the complete solution. That fix is in adcp-client, not this repo.

Pre-PR review

• code-reviewer: approved — all 6 structural checks pass (idempotency keys on all activate_signal steps, sample_request_skip_schema on intentionally-malformed step, REFERENCE_NOT_FOUND correct error code for unresolvable signal_agent_segment_id, model field correct for signals pricing)
• ad-tech-protocol-expert: approved — error codes correct per spec's "not-found precedence" rule (REFERENCE_NOT_FOUND for opaque signal_agent_segment_id, not SIGNAL_NOT_FOUND which is for signal_id catalog references); VERSION_UNSUPPORTED pattern via get_signals is sound; coverage sufficient to restore zero-step agents to participation

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See #3121
for context.

Session: https://claude.ai/code/session_01J2r8RXmLEsooTH82Q3XyY9

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