fix: update default system skill paths to .agents/skills

[DakshUbhadia]

Summary

This PR addresses the directory canonization mismatch reported in issue #420. The current public guidelines point users toward the newer ~/.agents/skills convention, but the default fallback behavior inside skill-installer and skill-creator still implemented and documented the legacy ~/.codex/skills directory.

Changes Made

• Updated _default_skills_dir() and _default_dest() fallback methods across init_skill.py, install-skill-from-github.py, and list-skills.py to point to ~/.agents/skills when $CODEX_HOME is unset.
• Preserved backward compatibility by maintaining structural evaluation of explicit $CODEX_HOME parameters.
• Updated argparse documentation/help descriptions to display the new pathing standards.
• Patched SKILL.md files to align execution behavior with the user-facing instruction sets.

Testing / Validation

• Successfully ran internal verification routines (quick_validate.py) across modified skill domains to ensure metadata schema schema validity (Skill is valid!).
• Verified execution pathways locally to confirm newly targeted directories load properly under Windows system structures.

Fixes #420