fix: deduplicate managed runtime skills

[chubes4]

Summary

• use .claude/skills as the canonical managed target when Claude Code and OpenCode are both detected
• remove only retired wp-coding-agents setup copies and noncanonical managed upgrade duplicates after a successful canonical install
• preserve OpenCode-only, Claude-only, explicit-runtime, user-skill, dry-run, and idempotent behavior with focused CI coverage

Closes #306.

Verification

• ./tests/managed-skill-targets.sh
• ./tests/codex-runtime.sh
• bash -n lib/skills.sh lib/summary.sh tests/managed-skill-targets.sh upgrade.sh setup.sh
• git diff --check