feat(plugin): pi agent support — one-liner install

[kellenff]

Adds a pi extension at repo root (multi-agent layout, parallel to the Claude plugin) that bootstraps yactt on first session and registers it with pi-mcp-adapter.

End user story

pi install git:github.com/kellenff/yactt

That's it. First session:

• plugins/yactt/scripts/install.sh fetches yactt (same TOFU + SHA256 chain as the Claude plugin).
• Installs npm:pi-mcp-adapter if not already present in ~/.pi/agent/settings.json.
• Writes a yactt MCP server entry to ~/.pi/agent/mcp.json (idempotent — preserves user-added servers).

Second session onward: no-op.

What's in this MR

• pi-extension/index.js — ~57 LOC extension
• pi-extension/package.json — declares pi.extensions entry
• pi-extension/test/extension.test.js — 4 smoke tests (52ms)
• skills/code-explore/SKILL.md — pi-format skill at repo root
• README.md + plugins/yactt/README.md — install section updated

Trust

Reuses the Claude plugin's scripts/install.sh unchanged — same SHA256 verification, same TOFU (version, sha256) record at ~/.local/share/yactt/known-good, same SLSA L3 attestations. The pi extension adds zero new attack surface; it just calls the existing installer and writes a JSON file.

Test

$ node --test pi-extension/test/extension.test.js
✔ session_start writes mcp.json with the yactt server
✔ session_start is idempotent and preserves user-added servers
✔ session_start tolerates a corrupt mcp.json
✔ ensureMcpAdapter is a no-op when settings already lists pi-mcp-adapter
ℹ tests 4
ℹ pass 4