feat(ai-core): Add Agent Skills support

[eneufeld]

What it does

Implements Agent Skills specification allowing AI agents to access reusable instructions and domain knowledge from SKILL.md files.

Skills are directories containing a SKILL.md file with YAML frontmatter (name, description) and markdown content. The {{skills}} variable resolves to XML listing available skills for injection into agent prompts.

Features:

• Skill discovery from configured directories and ~/.theia/skills/
• YAML frontmatter parsing with validation (name, description required)
• File watching for automatic skill updates
• {{skills}} variable contribution generating <available_skills> XML
• Duplicate skill detection with warnings (first directory wins)

New preference:

• ai-features.skills.skillDirectories: Additional skill directories

Closes #16762

How to test

Add some skill file as described in the readme.
Check that the agent sees the skills if you add {{skills}} variable to its prompt.

Follow-ups

For this feature to be really useful we need to be able to execute js and python (see #16772)

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

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