feat(ui): open skill source on GitHub

[terrytan95]

Description

Adds a direct GitHub source action to the skill overview header for skills pinned to GitHub. The renderer delegates opening the validated target URL through the existing Electron contract and places the action immediately before Delete Skill with the established button spacing.

Key Changes

• Show the GitHub action only for skills with a source pin.
• Resolve and open the exact pinned skill path through the main process.
• Add localized accessible labels and reuse the existing secondary action style.

Verification & Testing

• yarn typecheck
• yarn test — 41 tests passed
• yarn build:app
• Native Electron click opened the expected GitHub skill URL.

Impact & Risk

None.