Nixify pi

[o1lo01ol1o]

This PR add nix flake packaging to pi.

How to Use the Flake

From the repo root:

nix build path:$PWD#pi
nix run path:$PWD#pi -- --version
nix run path:$PWD#pi -- --help

Install the local flake build into your user profile:

  nix profile add path:$PWD#pi

After changing the checkout, refresh the installed profile entry:

  nix profile upgrade pi

Enter the development shell:

  nix develop --impure path:$PWD

Run validation:

  nix build path:$PWD#pi --no-link
  nix run path:$PWD#pi -- --version
  npm run check

The flake packages the core pi CLI only. Runtime state such as installed pi packages, extensions, settings, sessions,
and credentials remains outside the Nix store under the normal user/project locations like ~/.pi/agent and .pi/.

You can also install it to a nix profile directly from github:

nix profile add github:o1lo01ol1o/pi/feat/nix-flake-package#pi

Run it:

 pi --version
 pi --help

Upgrade later from the same branch:

 nix profile upgrade pi

For a specific commit:

 nix profile add github:o1lo01ol1o/pi/<commit-sha>#pi

[github-actions]

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Maintainers review auto-closed issues daily. Issues that do not meet the quality bar in CONTRIBUTING.md will not be reopened or receive a reply.

If a maintainer replies lgtmi, your future issues will stay open. If a maintainer replies lgtm, your future issues and PRs will stay open.

See CONTRIBUTING.md.