fix(agentplugins): reject unsafe and conflicting skill selections

[Whxuan0701]

Summary

• reject skill selections that are empty, traversal-like, or contain path separators
• reject duplicate skill names across standalone skills and selected plugin skills before fetching artifacts
• canonicalize existing and not-yet-created paths during containment checks so macOS /var aliases do not reject valid packages

This keeps Agent Plugin materialization deterministic and prevents one selected skill from overwriting another. It does not change the Agent Plugins schema or artifact source formats.

Testing

• cd go && go test ./core/v2/agentplugins ./adk/pkg/mcp ./core/v2/translator
• cd go && go vet ./core/v2/agentplugins
• git diff --check

Risk / Notes

• The new validation runs before any artifact is fetched or copied.
• Existing plugin and MCP behavior is unchanged for valid selections.
• The canonical-path handling preserves the existing symlink escape checks while making containment portable across macOS temporary-directory aliases.