pixi run install, install-docs and build-docs fail as documented: the task is ambiguous

The setup commands in CONTRIBUTING.md, README.md and AGENTS.md fail as written:

$ pixi run install
Error:   × the task 'install' is ambiguous
  help: These environments provide the task 'install': default, cuda, py311, py312, py313, py314

pixi run install-docs fails the same way, and so does pixi run build-docs, through its depends-on = ["install-docs"].

Cause

pixi.toml defines install and install-docs both in [tasks] and, with different commands, in [feature.cuda.tasks]. I isolated it on copies of pixi.toml at 607869e2, with every task command replaced by an echo:

This was on pixi 0.40.0, the version CI pins. pixi run install fails the same way on pixi 0.81.0 from curl -fsSL https://pixi.sh/install.sh | bash in a clean ubuntu:24.04 container, following CONTRIBUTING.md.

CI doesn't hit this because every workflow passes an environment: tests.yml uses pixi run -e <py env> install, and typecheck.yml uses pixi run -e default install.

Where it is documented

• CONTRIBUTING.md: lines 91 and 111 (pixi run install), 112 (pixi run install-docs), 137 and 188 (pixi run build-docs), and 161 ("Update it and run pixi run install").
• README.md: line 182 (pixi run install).
• AGENTS.md: lines 26 (pixi run install) and 44 (pixi run build-docs).

Scope

This is a maintainer choice between two approaches:

1. Rename the CUDA variants (for example install-cuda and install-docs-cuda) and update the one documented CUDA command, pixi run -e cuda install, to match. The bare commands then work as written everywhere.
2. Keep pixi.toml as it is and change every documented bare command to name the environment (pixi run -e default install, and so on).

Acceptance criteria

• Every pixi run command in CONTRIBUTING.md, README.md and AGENTS.md runs as written on pixi 0.40.0 and on the current pixi from install.sh.
• pixi run build-docs resolves its install-docs dependency.
• The CUDA environment still installs the CUDA build of torch.

Size: small. It's two lines in pixi.toml plus doc edits in three files, or doc edits only, and a check of each command on both pixi versions.

Found while setting up for #4628.