docs: record provider-account tenancy in changelog, fixtures and ADR edges

[rldyourmnd]

Summary

• CHANGELOG gains the missing [0.9.20] section documenting ADR-0040 tenancy, include[].match, policy match.name_prefixes and PLACEMENT_AMBIGUOUS retirement
• New schema fixture valid-device-include-match exercises include[].match (owner_login + name_prefixes + lifecycle + visibility) — closes the fixture-layer gap the Go tests already covered
• ADR index: 0040 now supersedes-in-part 0026/0028/0032 and those rows point back

Test plan

• python3 scripts/validate_gds_schemas.py — PASS (92 cases)
• scripts/validate_shell.sh — PASS
• scripts/validate_go_core.sh --quick — PASS
• pytest — 70 passed

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