Add tool entries for Just, Task, Pixi, Spin

[andrew]

Refs #61.

brief already parses Makefile, justfile, Taskfile.yml and package.json into the Scripts section, but Just, Task, Pixi and Spin had no tool entries so they never appeared under Build: or Environment: with a command hint.

This adds the four entries:

Build:       Just (just --list)  [justfile]
Build:       Task (task --list-all)  [Taskfile.yml]
Environment: Pixi (pixi install)  [pixi.toml]
Build:       Spin (spin)  [pyproject.toml]

Pixi goes under environment alongside Mise since it's primarily a conda-based environment manager; pixi task list and pixi run are in its alternatives. Spin is detected via [tool.spin] in pyproject.toml, a .spin.toml, or the dependency.

When a Taskfile or Justfile has a target named build the existing script-linking takes over and shows task build or just build instead of the list command, which is usually what you want.

This doesn't close #61 since the question there is broader; see the reply on that issue for the policy on running list commands.