docs(skills): v2 schema refresh — DEFAULT_REPORTING_CAPABILITIES + currency + affected_packages

[bokelley]

Follow-up to #716. Matrix v2 still 0/14 because upstream kept adding required fields. Redirects skills toward SDK helpers.

Fixes

1. DEFAULT_REPORTING_CAPABILITIES over hand-rolled literals (12 pairs) — seller, generative-seller, retail-media.
2. create_media_buy persists currency + total_budget flattened from request (6 pairs).
3. update_media_buy.affected_packages returns Package[] not string[] (6 pairs).

Still open

• SDK bug: si_get_offering context field collision (request string vs response object). Filing separately.
• Controller adoption (10 pairs still controller_detected: false). Iterating.

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