feat(environments): install the usage CLI so mise completions work - #106

Adds the usage CLI (5.0.0) to the environments role, so mise's shell
completions work.

Why

mise completion fish does not emit a completion script of its own. It emits
one generated by usage-cli, and that script's first act is to check for the
usage binary and give up:

# @generated by usage-cli from usage spec

# if "usage" is not installed show an error
if ! type -p usage &> /dev/null
    echo "Error: usage CLI not found. This is required for completions to work in mise." >&2
    return 1

roles/fish/files/fish/config.fish:158 regenerates that file into
~/.config/fish/completions/mise.fish on every interactive shell. So the
completion file exists and looks installed, and does nothing.

The gap is invisible to the obvious check: mise doctor reports "No problems
found", and it is right to — from mise's point of view nothing is wrong. It is
the generated completion script, not mise, that needs the binary.

Cross-platform, not Linux-only

No Brewfile declares usage, so the Macs have exactly the same dead
completions. apt has no usage package either. That leaves mise as the route on
both platforms, which is why this goes in roles/environments/ rather than
roles/linux/files/mise/linux.toml.

Declared in two places, deliberately

usage is added to both files/mise.toml and
defaults/main.yml's environments_mise_tools.

defaults/main.yml already carries a note about this, from when lua and
rust were added to mise.toml alone:

They must be listed for the mise ls --missing gate in tasks/main.yml to
converge: an undeclared but missing tool would keep the gate open forever
without ever being installed.

A mise.toml-only version of this change would leave mise ls --missing
reporting usage on every run while the install loop never reaches it, so the
role could never reach changed=0. That convergence is precisely what
specs/dev-services Task 7 has to demonstrate, so the one-file version would
have quietly blocked it.

Verification

• usage@5.0.0 installed on this host rather than trusting that the registry
  entry (aqua:jdx/usage) resolves. It does.
• yamllint, gitleaks, syntax-check, ansible-lint all pass.
• Checked that every entry in environments_mise_tools has a matching
  mise.toml entry, so this change does not reintroduce the lua/rust skew it
  cites.

Activation note

~/.config/mise.toml is a symlink into the primary checkout:

/home/inkatze/.config/mise.toml -> /home/inkatze/dev/dotfiles/roles/environments/files/mise.toml

So merging this and pulling in the primary checkout activates it directly. No
Ansible run is required for the config itself, and in particular no
mise run linux — which matters right now, because the power tasks in that
tag are currently able to deadlock the kernel on this host's Thunderbolt eGPU.
A fresh host still needs mise run environments to install the binary.