Move the agent CLIs to Nix, emptying mise's global tools

Move the agent CLIs to Nix, emptying mise's global tools claude, codex and pi-coding-agent come from nixpkgs now, so mise's global [tools] list is empty and everything global on this machine is managed by Nix. mise stays installed purely to serve the per-project mise.toml files. The self-update objection that kept them on mise was weaker than claimed: nixpkgs was one patch behind on claude-code (2.1.258 vs 2.1.259), one minor on codex, and exact on pi-coding-agent. Their in-place updaters cannot write to the read-only store, so they report being unable to update and defer to the package manager. They come from a second nixpkgs input that deliberately does not follow the first, so `nix flake update nixpkgs-agents` moves the three of them without dragging the other 59 packages. It needs its own import with allowUnfree because claude-code is unfree. Binary names verified: claude, codex, pi. Also corrects three stale README claims: the tool count, the ansible/boto3 status, and - the one that mattered - the note still saying Homebrew cleanup was "none" when it has been "zap" for several commits.