fix: allow deleting skills whose files were removed and 404 on enabling missing skills

[Georgyhongbo]

When an installed skill's files are removed outside the API, the dashboard kept a stale entry: DELETE reported "not found" so the entry could not be cleared, while "apply now" (enable) reported success without the skill existing.

• delete_skill is now idempotent: missing SKILL.md returns 204 so stale entries can be cleaned up.
• enable_skill validates the skill still exists in the catalog (workspace, builtin and mounted package skills) and returns 404 otherwise.

Summary

已安装技能的文件在页面外被手动删除后，网页上残留条目无法清理，且行为不一致：「删除」技能报「未找到」(404) 删不掉；同一技能点「立即应用」却提示「技能已启用」(204)，但技能文件根本不存在。

• delete_skill 幂等化：SKILL.md 缺失（已在页面外删除或已软删除）时返回 204，让前端能清掉残留条目。
• enable_skill 校验存在性：新增 _skill_catalog_exists，经 agent_registry.list_skill_summaries 实时读盘，覆盖 workspace / builtin / 挂载的全局技能包；技能不存在时返回 404，不再误报「已启用」。
• 补充集成测试：enable 未知技能 404、enable 已删除技能 404、delete 未知技能 204（幂等）、delete 已删除技能两次均 204。
• 改完后：

Target branch

• Base is develop (feature / fix — default)
• Base is main (release/* or hotfix/* only)

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactor / chore
• Release / hotfix

Test plan

• make all passes locally
• Added/updated tests

实际验证：

• tests/integration/test_skills_api.py → 28 passed
• 相关技能套件（unit/agents、unit/skills、cli skills、skill packages 集成）→ 288 passed / 3 skipped
• 前端无需改动：apiErrorMessage 已会展示服务端 404 detail

Checklist

• Updated CHANGELOG.md (if user-facing)
• README / docs updated (if needed)