Add Pi Coding Agent (pi.dev) integration

[martian7777]

What does this PR do?

Adds support for the Pi Coding Agent to the Agency integrations.

• Conversion (scripts/convert.sh): Added convert_pi() to generate Pi-compatible SKILL.md files. Each agent is placed into its own skill directory with the agency- prefix (e.g., agency-frontend-developer) to prevent namespace collisions with existing Pi skills.
• Installation (scripts/install.sh): Added the pi tool target, complete with automatic detection (looks for ~/.pi), interactive wizard support, and bulk installation of skills to the standard ~/.pi/agent/skills/ directory (with PI_SKILLS_DIR override support).
• Documentation: Created integrations/pi/README.md containing usage instructions for Pi, and updated integrations/README.md to list the new tool.
• Git Ignore: Updated .gitignore to prevent generated Pi skills (integrations/pi/agency-*/) from being committed, following the project's convention for generated files.
• fix the issue of Pi.dev support #470

Agent Information (if adding/modifying an agent)

• Agent Name: N/A
• Category: N/A
• Specialty: N/A

Checklist

• Follows the agent template structure from CONTRIBUTING.md (N/A - Integration script update)
• Includes YAML frontmatter with name, description, color (N/A)
• Has concrete code/template examples (for new agents) (N/A)
• Tested in real scenarios (Converted and dry-run installed locally)
• Proofread and formatted correctly