feat: add icons for custom agent folders and files

[marlondecol]

Description

This pull request introduces new icons for custom agent files and folders, for both Copilot and Claude, following the default location configuration for those files. The icons are derived from the icon used by VS Code itself in agent-related contexts, e.g., as it appears in the Chat's Agents dropdown and the Agent Customizations panel. Even though it is not a Material Design icon, that is why I considered it appropriate to adopt it for these new icons.

Codicons agent icon

The agent files for both Copilot and Claude are associated with the copilot-agent icon, which also has a light-theme variant. The folder for Copilot is associated with the folder-copilot-agent icon, but the folder for Claude is associated with an orange clone of that icon. I would have done the same for Claude agent files, but it is not yet possible to associate an icon with file paths that have more than one parent (see the docs), and both files share the chatagent language ID.

Other Changes

This pull request also creates a light-theme variant for the github-actions-workflow icon, so that it matches the appearance of the copilot-agent icon. This improves the icon's contrast against both light and dark backgrounds.

Additionally, it refactors the folder-github and folder-gh-workflows icons so that their motives use colors from the second row of the respective palette, as required by the Contribution Guidelines. This adjustment also slightly improves the contrast of those elements against light backgrounds.

Finally, the markdown icon is disassociated from the md and markdown extensions to avoid conflicts with the other changes. The markdown language ID already accounts for those extensions, so those associations are unnecessary.

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Feel free to suggest improvements!

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