fix: reject duplicate skill sources

[calvinmclean]

Addresses #7272

[Copilot]

Pull request overview

This PR addresses issue #7272 by enforcing uniqueness for Skill Sources (Skill Repositories) on both the backend API and UI, preventing duplicate display names and duplicate repository URLs and surfacing clearer validation errors in the admin Skills page.

Changes:

• Add server-side validation to reject creating/updating a Skill Source when another source already uses the same displayName or normalized repoURL (409 Conflict).
• Improve admin UI error display by parsing API error payloads/messages into a user-facing message.
• Add handler tests covering duplicate-name and duplicate-URL rejection, and ensuring updates can keep their own name/URL.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ui/user/src/routes/admin/skills/+page.svelte	Uses parseErrorContent() so validation/conflict errors are shown as clean user-facing messages.
pkg/api/handlers/skills_test.go	Adds coverage for duplicate Skill Source name/URL rejection on create/update, and allows self-update.
pkg/api/handlers/skillrepositories.go	Adds validateUniqueSkillRepository and wires it into create/update flows to enforce uniqueness.

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