Agent plugin skill namespaces are dropped, causing same-named skills to shadow each other

- Copilot Chat Extension Version: 0.64.1
- VS Code Version: 1.136.1 (`a44adf7f53e00964ab890f9f8758a334f1fc15bc`, arm64)
- OS Version: macOS 26.5.2 (`25F84`), arm64
- Feature (e.g. agent/edit/ask mode): Agent mode
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): `gpt-5.6-sol`
- Logs: Redacted session debug log and system-prompt excerpt attached.

## Summary

When multiple installed Copilot agent plugins provide skills with the same
`name` in `SKILL.md`, only one of those skills is included in the model context.

Plugin discovery initially identifies the skills using plugin-qualified names,
but the qualification is lost during final skill loading. The skills are then
deduplicated by their unqualified name, causing one plugin's distinct skill to
be dropped.

## Steps to Reproduce

1. Install and enable two agent plugins that each contain a skill such as:

- `plugin-a/skills/troubleshooting/SKILL.md`
- `plugin-b/skills/troubleshooting/SKILL.md`

Both files use:

```yaml
---
name: troubleshooting
---