Add review-gated social campaign handoff skill

[szmyty]

Closes #53

Summary

• define the aether.social-campaign-handoff/v1 contract and lifecycle
• add the portable prepare-social-campaign-handoff skill and deterministic CLI
• lock packets to exact Identity package and Aether catalog digests
• require freshness evidence, checklist completion, human review, and digest-bound approval before immutable export
• keep publishing behind a separately authorized adapter with a receipt bound to the exported packet
• reject credentials, direct-post authority, unsupported surfaces, invented approvals, stale inputs, and missing attribution
• include organization-announcement and product-release examples plus schema and adversarial fixtures

Validation

• python3 -m unittest tests.test_social_campaign_handoff --verbose
• python3 library/organization/skills/validate-skills.py
• python3 library/organization/skills/build-distributions.py --output-directory "dist" --check
• python3 -m unittest tests.test_build_distributions tests.test_build_spec_distributions --verbose
• git diff --check

The local runtime did not include the pinned jsonschema dependency, so the formal schema test was skipped locally and is left to the repository's deterministic CI environment.