#898 repo upgrade shadowing warning should list all shadowed skills by name

Description

During swamp repo upgrade, the warning about locally shadowed skills is potentially misleading because it does not make clear which specific skills are being shadowed before the upgrade runs.

Steps to Reproduce

1. Have a repo with several swamp skills checked in under .claude/skills/ (e.g. swamp-data, swamp-extension, swamp-model, etc.)
2. Run swamp repo upgrade

Observed Behavior

A pre-upgrade warning lists 10 skills as "superseded":

warning swamp·cli  "10 superseded skill(s) found: swamp-data, swamp-extension, swamp-extension-publish, swamp-issue, swamp-model, swamp-repo, swamp-report, swamp-troubleshooting, swamp-vault, swamp-workflow. Run 'swamp repo upgrade' to update skills."

After the upgrade, the shadowing warning only names one:

warning repo·upgrade  Local swamp skill copies are shadowing the globally installed skills. Delete them manually:
warning repo·upgrade    /Users/dmc/projects/ai-infra/.claude/skills/swamp-getting-started
info    repo·upgrade  Skills updated: swamp, swamp-getting-started

This is confusing because:

• The pre-upgrade "superseded" warning implies all 10 local skills are outdated copies of global skills.
• In reality, only swamp-getting-started (and swamp) are globally installed — the other 10 are project-local skills that have no global counterpart.
• A user reading this could incorrectly conclude that all 10 local skills are shadowing global ones and delete them, losing skills (like docker and proxmox) that only exist locally.

Expected Behavior

The post-upgrade shadowing warning already lists paths, which is good. The pre-upgrade "superseded" warning should either:

• Clarify that it refers to locally-checked-in swamp-managed skills (not all local skills), or
• Not include skills in the count/list that don't have a corresponding global install

This would prevent users from mistakenly thinking all their local skills need to be deleted.