feat: signal activation execution paths and testing storyboards

[bokelley]

Summary

Addresses #1949 (signal activation DSP discovery and execution path), which consolidated three spec gaps surfaced by joohwa kim (NHN ACE) during A1 certification (#1944, #1945, #1946).

• Spec clarification: Documents the two signal activation patterns — platform destinations (buyer manages DSP targeting) and agent destinations (SA handles downstream coordination). No schema changes needed; the protocol already supports both patterns, the docs just didn't explain when to use each.
• Signal agent storyboards: Adds two testing storyboards for signal agent builders (marketplace and owned), plus a Nova Motors test kit with sample signals.

Changes

Spec docs (3 files):

• activate_signal.mdx — "Using Activation Keys" section with execution paths
• overview.mdx — Sales agent activation example in the walkthrough
• specification.mdx — "Destination Type Selection" conformance language

Signal storyboards (3 new files):

• signal_marketplace.yaml — 4 phases: discovery, catalog verification, platform activation, agent activation
• signal_owned.yaml — 3 phases: discovery, platform activation, agent activation
• test-kits/nova-motors.yaml — 4 marketplace + 4 owned signals, platform and agent destinations

Supporting changes:

• schema.yaml — Extended category/interaction_model enums for signals
• storyboards.test.ts — 12 new assertions covering signal storyboard structure

Test plan

• npx vitest run server/tests/unit/storyboards.test.ts — 32/32 pass
• Full test suite via pre-commit hook — 567/567 pass
• TypeScript typecheck passes
• Mintlify link validation passes (pre-push hook)
• Verify spec pages render correctly on preview deploy

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