feat: align marketplace plugin compatibility

[gzb1128]

Summary

• add JSON output for market/plugin list and marketplace command alias
• support displayName metadata and pinned marketplace refs
• add CLAUDE_PLUGIN_DATA substitution with persistent per-plugin data dirs
• clean orphaned plugin cache versions after update
• document the compatibility-layer intent in AGENTS.md

Verification

• GOCACHE=/private/tmp/codex-gocache go test ./...
• GOCACHE=/private/tmp/codex-gocache go vet ./...
• GOCACHE=/private/tmp/codex-gocache go build -o bin/opencode-plugin .
• git diff --check