feat: add support for .agents/skills directory for project skills

[daiv-agent]

This PR adds support for the .agents/skills directory as an additional location to host project-specific agent skills. It updates constants, documentation, and tests to recognize and load skills from this new directory alongside existing ones.

Key Changes:

• Added .agents/skills path to the list of skill sources in constants.py.
• Updated README and documentation to mention .agents/skills as a valid skills directory.
• Modified skills middleware tests to verify discovery of skills from .agents/skills along with other skill directories.
• Updated changelog to document the new .agents/skills support.

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Closes: #787

⚠️ DAIV can make mistakes. Please review the changes and merge the MR if everything looks good.

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• 📝 Edit the original issue (Support .agents/skills dir alias #787) to get DAIV to recreate the MR from scratch.

[srtab]

@daiv-agent fix the pipeline

[daiv-agent]

Fixed the pipeline - formatting error in the test file has been corrected. All linting checks and tests now pass.