Add install command

[codeaholicguy]

Summary

Adds a new ai-devkit install command to apply project setup from .ai-devkit.json in one run, with safer overwrite behavior, structured config loading/validation, and improved test coverage/docs.

What changed

CLI

• Added install command to CLI:
  • ai-devkit install
  • ai-devkit install --config
  • ai-devkit install --overwrite

Behavior details

• Without --overwrite:
  • Detects existing install artifacts.
  • Prompts once to overwrite all conflicts.
  • If declined, conflicting env/phase artifacts are skipped.
• With --overwrite:
  • Overwrites all conflicts automatically without prompt.
• Skill install failures:
  • Reported as warnings.
  • Do not force non-zero exit code by themselves.
• Exit code:
  • 1 for invalid/missing config, or env/phase failures.
  • 0 otherwise (including partial skill failures).