Fix registry:migrate parity gaps and bugs vs hc (#207)

[BlueLabelscotch]

Summary

Closes #207. Brings harness execute registry:migrate to parity with hc's registry migrate, fixing all reported bugs and adding all missing features.

Bugs

• 1. Failed artifacts don't fail the run — Run() now tallies StatusFail records and joins a %d of %d artifact(s) failed to migrate error into the returned error, so any failure exits non-zero instead of printing "Migration completed successfully."
• 2. packageFilters silently dropped — added packageFilters to Config (types/selector.go) with per-artifact-type granularity (util/selector_util.go), applied during registry/package traversal.
• 3. Unknown artifactType not rejected at config load — validateConfig now calls IsKnownArtifactType, rejecting unknown/mistyped types before any network calls.

Missing features

• 5. --result-file — writes one JSON-lines record per artifact (status, error, skip reason) to a configurable path, written before the exit-code decision so automation sees the full picture even on failure.
• 6. --summary flag — prints condensed Success/Skipped/Failed/Total counts instead of the full per-file table.
• 7. Missing artifact types (CRAN, RUBY, TERRAFORM) — full adapter/migratable/test support added for all three
• 8. No skip reason — FileStat now carries a Reason field (already_exists, no_content) populated at every skip site, so it's clear why an artifact was skipped.
• 9. Config file not documented in help — added a long help block to execute registry:migrate covering an example config, supported artifact types, packageFilters granularity, and include/exclude pattern behavior.