feat(spec): add sponsored-intelligence to AdCPSpecialism enum

[bokelley]

Closes #3961

Adds sponsored-intelligence as a preview specialism in AdCPSpecialism, closing the asymmetry between the SI protocol declaration (supported_protocols: ['sponsored_intelligence']) and the specialism registry. The four SI lifecycle tasks (si_get_offering, si_initiate_session, si_send_message, si_terminate_session) have been in the spec since 3.0 — this gives them a specialism ID so SDK v6 can define SponsoredIntelligencePlatform using the same RequiredPlatformsFor<S> dispatch pattern as all other specialisms.

Non-breaking justification: Adds a new enum value appended to the end of AdCPSpecialism; existing agents are unaffected; specialism claims are opt-in; the protocol declaration (sponsored_intelligence in supported_protocols) continues to coexist unchanged.

Three-location atomic update required to satisfy the adcp-taxonomy.test.ts parity check:

1. static/schemas/source/enums/specialism.json — new enum value + enumDescriptions entry
2. server/src/services/adcp-taxonomy.ts — new AdcpSpecialism union member + ADCP_SPECIALISMS array entry
3. static/compliance/source/specialisms/sponsored-intelligence/index.yaml — new preview storyboard with requires_scenarios: [si_baseline]

Ships as status: preview because the underlying SI schemas carry x-status: experimental. The compliance runner produces no stable pass/fail verdict until schemas graduate. Consistent with the preview-specialism pattern established for other experimental surfaces. Fits the #2511 (re-add preview specialisms) wave in 3.1.

The storyboard has a single capability_discovery phase verifying both sponsored_intelligence in supported_protocols and sponsored-intelligence in specialisms. The full SI session lifecycle (offering discovery → session lifecycle) is pulled in via requires_scenarios: [si_baseline]. This is intentional for a preview storyboard — the lifecycle is already covered by the protocol-level baseline; the specialism adds the coexistence invariant check.

Nit (not addressed here): The protocol-level SI storyboard at static/compliance/source/protocols/sponsored-intelligence/index.yaml uses real company names in agent.examples which violates the playbook's no-real-brands rule. Separate cleanup warranted — not in scope for this enum-addition PR.

Pre-PR review:

• ad-tech-protocol-expert: approved — non-breaking per spec; one blocker found and fixed (requires_scenarios must reference storyboard id: si_baseline, not filesystem path protocols/sponsored-intelligence)
• code-reviewer: approved — one blocker found and fixed (stale comment + missing preview assertion in adcp-taxonomy.test.ts); remaining findings are nits or already-resolved (the requires_scenarios path fix was applied before their review ran)

Triage-managed PR. This bot does not currently iterate on review comments or PR conversation threads (only on the source issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> → fix → push.
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See #3121 for context.

Session: https://claude.ai/code/session_015vM78ky3xSqQj4FMqEZmDe

[bokelley]

Closing in favor of #3964, which is the leaner implementation of the same enum addition (sponsored-intelligence → AdCPSpecialism). Tracking continues on #3964.