Proposal: add Pi coding agent installer integration

[EgorYolkin]

Proposal

Add Pi as a supported target in scripts/install.sh.

Pi does not provide built-in subagents, but it supports Agent Skills as Markdown files. Agency agent files already contain the required name and description frontmatter, and Pi discovers root .md skills in ~/.pi/agent/skills/. This means the installer can expose Agency agents to Pi as skills without a separate conversion step.

Proposed behavior

• Detect Pi through the pi executable or ${PI_CODING_AGENT_DIR:-~/.pi/agent}.
• Install selected Agency agent Markdown files into ${PI_CODING_AGENT_DIR:-~/.pi/agent}/skills/.
• Preserve existing installer behavior for --division, --agent, --agents-file, --link, --path, --dry-run, and parallel installation.
• Ask users to run /reload or restart Pi after installation.
• Document full installation and isolated single-skill testing.

Example:

./scripts/install.sh --tool pi --agent code-reviewer --dry-run
./scripts/install.sh --tool pi --agent code-reviewer --no-interactive

The installed skill can then be invoked in Pi with:

/skill:code-reviewer

Implementation considerations

Per CONTRIBUTING.md, a complete integration should update the tool registry and consistency surfaces, including tools.json, scripts/check-tools.sh expectations, integration documentation, and any required conversion or shared-format registration. Since Pi can consume the source Markdown directly, I would like feedback on whether maintainers prefer:

1. a source-only installer integration with no generated files; or
2. reuse of the existing skill-md conversion format and a conventional integrations/pi/ target.

Draft implementation

A draft PR is available for early review: #736

I have kept it as a draft pending alignment on the expected integration shape.