Duplicate skills load from repository and installed plugin

Problem

Copilot CLI loads the same skill twice when a repository contains a project skill and an enabled user plugin contributes the identical skill. This doubles the skill description surface in repositories that develop their own plugin.

Observed with Copilot CLI 1.0.79-9 in rjmurillo/ai-agents, where .claude/skills is the canonical source for the installed project-toolkit plugin.

Reproduction

cd <repo-that-contains-the-plugin-source>
copilot plugins list --json | jq -r '\n[.plugins[] | select(.kind=="skill") | {name,scope,source,description:(.description // "")}]\n| group_by(.name)[]\n| select((map(.scope)|index("repository")) and (map(.scope)|index("plugin")))\n| [. [0].name, (length|tostring), ([.[].scope+":"+.[].source]|join(","))]\n| @tsv'\n```\n\nMeasured result:\n\n```text\nduplicate_names=97\nidentical_descriptions=97\nduplicate_description_bytes=40624\n```\n\nExample name:\n\n```text\ncontext-optimizer  plugin  plugin\ncontext-optimizer  repository  project\n```\n\nThe installed plugin is enabled at user scope. No repository-scoped setting was found to disable that one plugin only in its source repository.\n\n## Expected\n\nWhen a repository skill and plugin skill have the same canonical name and identical content, load one entry. Project scope could take precedence, or the CLI could deduplicate by name plus content hash. A repository-scoped plugin-disable override would also provide a workaround.\n\n## Control\n\nThis is specific to skills. A canary placed in `plugin/instructions/*.instructions.md` did not load through `--plugin-dir`, while a repository `.github/instructions` canary did load.\n

首先，在包含 .claude/skills 和一个已启用 plugin 的 repository 中使用 copilot plugins list --json 复现问题，然后追踪 repository 和 plugin skill 的发现过程。比较 repository 和 plugin 条目的规范名称和内容，并验证相同的 skill 会生成一个已加载条目，且不会影响文档中记录的 instruction canary。