improvement: installer registers arm_commands handlers in generated robot

[jimsynz]

Summary

mix igniter.install bb_so101 now adds the arm_commands example package
as a sparse git dep against
beam-bots/bb_examples, and
registers the three demo command handlers as commands on the generated
robot — home, move_to_pose, and demo_circle — each with full DSL
argument metadata so the bb_liveview dashboard renders proper input
widgets for every goal field.

The user gets a fully demo-ready arm out of a single
mix igniter.install bb_so101: IK-driven moves to a Cartesian target, a
configurable demo circle in any of the three axis-aligned planes, and a
parameterised home command.

What gets generated

defp deps do
  [
    {:arm_commands,
     git: \"https://github.com/beam-bots/bb_examples.git\",
     sparse: \"arm_commands\"}
    # …
  ]
end

commands do
  command :arm,           do: handler(BB.Command.Arm)
  command :disarm,        do: handler(BB.Command.Disarm)

  command :home do
    handler BB.Examples.ArmCommands.Home
    allowed_states [:idle]
    argument :position, :float, default: 0.0, doc: \"…\"
  end

  command :move_to_pose do
    handler BB.Examples.ArmCommands.MoveToPose
    allowed_states [:idle]
    argument :ee_link, :atom, default: :ee_link, doc: \"…\"
  end

  command :demo_circle do
    handler BB.Examples.ArmCommands.DemoCircle
    allowed_states [:idle]
    argument :ee_link, :atom, default: :ee_link, doc: \"…\"
    argument :plane, {:in, [:xy, :xz, :yz]}, default: :xz, doc: \"…\"
    argument :radius, :float, default: 0.03, doc: \"…\"
    argument :points, :integer, default: 16, doc: \"…\"
    argument :settle_tolerance_m, :float, default: 5.0e-3, doc: \"…\"
    argument :settle_timeout_ms, :integer, default: 1500, doc: \"…\"
  end
end

Implementation note

There's no BB.Igniter.add_command/4 helper in bb yet, so the
installer carries its own small add_command/4 private function that
wraps Spark.Igniter.update_dsl the same way BB.Igniter.add_controller/4
already does internally. Idempotent on command name. If/when more
installers need this, it can be promoted into BB.Igniter.

Test plan

• mix check --no-retry green
• 4 new tests cover: sparse git dep added to mix.exs; each of the
  three commands registered with the right handler and at least one
  representative argument declaration.
• Existing idempotency test still passes — running the installer
  twice produces no further changes.