XY-690: Port Radar ledger and GitHub artifact utilities to decodex radar

[yvette-carlisle]

Summary

• Add decodex radar ledger ... commands for bootstrap, ingest, ingest-existing, artifact-link, and summary.
• Add decodex radar bundle build and decodex radar bundle validate for deterministic GitHub change bundles.
• Preserve ledger schema semantics, including social_draft to social_post migration, and update docs for command ownership.

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make test
• cargo make decodex-checks
• cargo test -p decodex radar::tests
• cargo run -p decodex --bin decodex -- radar bundle validate artifacts/github/bundles
• cargo run -p decodex --bin decodex -- radar ledger --db <temp> ingest-existing --bundles-dir artifacts/github/bundles --analysis-dir artifacts/github/analysis --signals-dir site/src/content/signals