chore(engine): adopt AsDecided repository identities

[tcballard]

Summary

• migrate active repository, release, docs, and action references from rac-core to asdecided-core
• point native CI and contract documentation at asdecided-spec
• point reusable CI examples at asdecided-ci
• update companion repository references and future OCI publication to canonical AsDecided names
• preserve stable RAC-* artifact IDs and compatibility fixtures

External state already verified

• asdecided-core, asdecided-spec, asdecided-ci, asdecided-sdk, asdecided-connectors, asdecided-editors, and asdecided-benchmarks are live
• legacy Git URLs resolve to the same HEAD commits
• v0.23.1 release assets and Actions history survived the core rename

Verification

• authoritative registry sync: 5 specs, 9 relationship descriptions
• language-neutral conformance: 11/11
• Rust release workspace suite: pass
• core corpus validation: 427/427 known artifacts valid; 11 unknown skipped
• relationship validation: pass
• gate: pass, 0 blocking findings
• MkDocs strict build: pass
• modified JSON documents: parse successfully
• git diff --check: pass

cargo fmt --check is not a useful gate for this metadata-only change because the current local rustfmt wants to reformat thousands of untouched baseline lines; no Rust implementation formatting was changed.