docs(signals): add Inspector verification steps to Signals tutorials

[jborlase-snowplow]

Summary

The Snowplow Inspector's new Signals integration provides live Attributes and Interventions tabs in DevTools, but several Signals tutorials still verified attributes only via notebook/SDK pulls or console.log. This PR makes the Inspector the first, zero-code verification step across those tutorials, and fixes two bugs on the Signals integration page itself.

• tutorials/signals-quickstart/retrieve-attributes.md: restructured so the first verification step is the Inspector's Attributes tab, with the Python SDK retrieval kept as the "use it in your application" step. The Colab notebook remains the vehicle for the SDK step, so meta.json is unchanged.
• tutorials/signals-ai-agent-context/test.md: added an Inspector verification section (Events tab for tracking, Attributes tab for computed values) before the console.log fallback.
• tutorials/signals-google-adk-agent/build-agent.md: the "Verify Signals context" section now leads with the Inspector before the agent-log check.
• tutorials/signals-ml-prospect-scoring/define-prospect-attributes.md: added Attributes-tab verification after publishing, before wiring up the ML model.
• docs/testing/snowplow-inspector/signals-integration/index.md: the Interventions screenshot now embeds the previously unused signals-interventions.png (it duplicated the Attributes screenshot), and the dangling sentence "...from the list of received interventions on the will display" is repaired.

Checks

• All new internal links verified to exist (/docs/testing/snowplow-inspector/signals-integration/ etc.)
• Attribute names in the quickstart verification step match those defined in define-attribute-group.md

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