feat(library): add open-folder action for skills in management page

[Pleasurecruise]

What this PR does

Before this PR:

Skills in Cherry Studio live as folders on disk (SKILL.md plus scripts/assets), but the Skills management page offered no way to get from a skill to its folder — users had to hunt for the install location manually to share, back up, or zip a skill.

After this PR:

Each skill's card menu on the Skills management page gains an Open folder action that reveals the skill's on-disk directory in the OS file manager (Finder on macOS, File Explorer on Windows), with the skill's folder selected/highlighted. From there users can copy/zip the real skill directory directly.

Fixes #15955

Why we need it and why it was done in this way

The skill folder lives at feature.agents.skills ({userData}/Data/Skills/<folderName>) in the main-process path registry. Rather than add a new IPC channel, the action reuses existing renderer APIs end-to-end:

• window.api.getAppInfo() → appDataPath
• window.api.resolvePath() to normalize the path cross-platform
• window.api.file.showInFolder() to reveal it (this already does an existence check and calls shell.showItemInFolder, which highlights the folder)

showInFolder throws when the folder doesn't resolve, so the negative case surfaces a toast instead of a broken/erroring action.

The following tradeoffs were made:

• The renderer builds the path as ${appDataPath}/Data/Skills/<folderName>, which hardcodes the skill-storage subpath that otherwise lives only in the main-process path registry (feature.agents.skills). This keeps the change purely renderer-side with zero new IPC surface, at the cost of a silent break if that registry path ever moves. An alternative (exposing skillsPath via App_Info, or a dedicated Skill_OpenFolder IPC) was considered and intentionally not taken to keep the change minimal.

The following alternatives were considered:

• Adding a dedicated Skill_OpenFolder IPC (resolve + reveal in main) — rejected as redundant given the existing reveal IPC.
• Adding skillsPath to App_Info — rejected to avoid widening the app-info payload.

Links to places where the discussion took place: N/A

Breaking changes

None.

Special notes for your reviewer

The action is shown for every skill in the management list (all are installed on disk). Built-in skills (e.g. skill-creator) are intentionally included, matching the issue's verification example.

Checklist

• Branch: This PR targets the correct branch — main for active development, v1 for v1 maintenance fixes
• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Documentation: A user-guide update was considered and is present (link) or not required. Check this only when the PR introduces or changes a user-facing feature or behavior.
• Self-review: I have reviewed my own code before requesting review from others

Release note

Skills management: add an "Open folder" action to reveal a skill's folder in Finder / File Explorer for easy sharing or backup.