Fix add-skill limbo when SKILL.md is manually deleted

[AndrewAsseily]

Description of changes:

add-skill previously got stuck when a previous install had its SKILL.md deleted by hand but its .aws-skill-metadata marker left behind. The marker made the toolkit treat the skill as already installed, but the skill was actually unusable. This change detects the corrupted state (marker present, SKILL.md missing) and reinstalls the skill.

Testing:

aws-cli % rm -rf /Users/.../.claude/skills/aws-cdk/SKILL.md
aws-cli % aws agent-toolkit add-skill --skill-name  aws-cdk --region us-east-1
  Reinstalling aws-cdk at /Users/.../.claude/skills/aws-cdk: previous install appears incomplete.
  Installed aws-cdk (v1) to Claude Code — ~/.claude/skills.
  aws-cdk is already installed (v1) at /Users/.../.kiro/skills/aws-cdk. Run "aws agent-toolkit update-skill --skill-name aws-cdk" to update, or remove it first to reinstall.

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