fix(skills): preserve npm/nuget version ranges in dependency upgrades

[brianhdk]

Summary

• Update upgrade-dependencies skill instructions to preserve existing npm and NuGet version ranges during upgrade flows.
• Add explicit guidance to skip range-based declarations (for example npm comparator ranges and NuGet bracket/parenthesis ranges) instead of rewriting them.
• Require reporting of skipped ranged dependencies and recommended minimum-version bumps in both PR summary guidance and final output expectations.

Scope

• Skill documentation only: .agents/skills/upgrade-dependencies/SKILL.md.
• No runtime code, config, or dependency lock updates.