chore(dev-tools): rely on global soldr instead of pyproject dependency

[zackees]

Context

The FastLED org should rely on a globally installed soldr rather than pulling
soldr into repo-local Python/dev-tool environments.

Repository scan result:

• FastLED/fbuild contains ci/dev-tools/pyproject.toml
• That file currently declares dependencies = [soldr>=0.7.4]
• ci/dev-tools/README.md says the package provides the soldr dependency
• ci/trampoline.py tells users to run ./install or uv sync to install
  fbuild-dev-tools, which pulls in soldr

This means soldr can be resolved from the repo-local Python environment instead
of the global tool installation. That makes agent and developer behavior less
consistent across repos.

Proposal

Remove soldr from ci/dev-tools/pyproject.toml and require soldr to be
available globally on PATH.

When soldr is missing, the error should clearly say that soldr must be
installed globally and point users to the upstream install options, for example:

• uvx / uv-based install path
• script install instructions from https://github.com/zackees/soldr

Update nearby docs and helper error text so they no longer say that uv sync or
fbuild-dev-tools installs soldr as a dependency.

Acceptance criteria

• ci/dev-tools/pyproject.toml no longer declares soldr as a dependency.
• Helper code still runs soldr from PATH.
• Missing-soldr errors explain that global soldr is required.
• Missing-soldr errors recommend uv/uvx or script install and link to
  https://github.com/zackees/soldr.
• ci/dev-tools/README.md no longer claims the repo-local package provides the
  soldr dependency.