[Bugfix] explain: REASON text masks real rejection cause

[pallasathena92]

What this PR does

Post-merge review of #759 confirmed two defects in kubectl ome runtime explain's REASON column (COMPATIBLE verdicts were always correct):

• A non-matched runtime that passes ValidateRuntime got a hardcoded "no autoSelect entry" reason — factually wrong when the real cause is a non-auto-selectable matching format or an explicit priority-0 (zero score) drop. The reason is now recomputed from the actual per-format auto-select/score semantics.
• ValidateRuntime resolves by name only (namespaced shadows cluster), so a cluster-scoped candidate sharing a name with a namespaced runtime had its rejection reason computed against the wrong object. Same-name rows are now evaluated against the correct scope's spec.

Why we need it

The whole point of runtime explain is trustworthy "why" output; a wrong reason is worse than none. Follow-up to #759 (OEP-0011).

How to test

go test ./pkg/cli/cmd/runtime/...

Checklist

• Tests added/updated
• Docs updated (N/A)
• Scoped test suite passes locally (go test ./pkg/cli/... — full make test requires the Rust xet toolchain, unaffected by this PR)