Add pi with a lean global workflow to the devcontainer

[Copilot]

Bring the reusable parts of the proposed pi setup into the devcontainer, keeping approvals-specific rules and squad configuration project-local.

• Installation: Pin pi to 1.1.0 and i-have-adhd to a commit; configure always-on mode independently for node and root.
• Global workflow: Bundle short preferences, four skills for spec → plan → implement → verify/debug, and /spec, /plan, /slice prompts. Use each project's existing checks rather than assuming mise.
• Startup verification: Check pi execution, bundled resources, and user-scoped package registration and installed files.
• Documentation: Cover manual login/model selection, persistent configuration, and optional project-local pi-squad with a human review gate.

No authentication, approvals stack rules, or superpowers package is baked into the image.