[minion] Detect external hook managers during enable

[jcleira]

Objective

During partio enable, detect if external Git hook managers (Husky, Lefthook,
Overcommit) are already installed in the repository. If detected, warn the user
about potential conflicts with Partio's own hook installation.

Detection should check for:

• Husky: .husky/ directory or prepare script in package.json
• Lefthook: lefthook.yml or .lefthook.yml in repo root
• Overcommit: .overcommit.yml in repo root

The warning should be informational (not blocking). Suggest running partio doctor
for more details.

Acceptance Criteria

• partio enable warns when Husky .husky/ directory is detected
• partio enable warns when lefthook.yml is detected
• partio enable warns when .overcommit.yml is detected
• Warning is informational, does not block hook installation
• make test passes in cli/
• make lint passes in cli/
• docs/cli/commands.mdx updated with hook conflict info if applicable

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Automated PR by partio-io/cli · Task: detect-external-hooks-cli-agent

Created by an unattended coding agent. Please review carefully.