feat: add Cheer.argv/0 for Burrito-aware argv

[joshrotenberg]

Problem

A Burrito-wrapped binary does not populate System.argv/0. Arguments arrive
through Burrito.Util.Args.argv/0 instead, and Burrito.Util.running_standalone?/0
is what tells you which mode you are in. Every Cheer app that ships via Burrito
therefore writes the same shim:

defp argv do
  if Code.ensure_loaded?(Burrito.Util) and Burrito.Util.running_standalone?() do
    Burrito.Util.Args.argv()
  else
    System.argv()
  end
end

tinfoil_demo hand-writes it. hexpm-mcp is about to write the same thing
(joshrotenberg/hexpm-mcp#68). Anyone using the burrito + tinfoil + cheer stack
hits it.

The failure mode when you get it wrong is silent rather than loud: System.argv/0
returns [], every option falls through to its default, and the binary does
something plausible but wrong. In hexpm-mcp's case --transport stdio would
have fallen through to the :http default and started a web server while the
MCP client waited forever for a stdio handshake.

Proposal

@spec argv() :: [String.t()]
def argv()

Returns Burrito.Util.Args.argv/0 when running as a standalone Burrito binary,
System.argv/0 otherwise.

Cheer keeps its zero-runtime-dependency property by resolving Burrito through
Code.ensure_loaded?/1 rather than taking a dep.

Notes

Cheer.main/3 should probably default to this rather than requiring the caller
to pass argv in, though the explicit-argv form has to keep working for tests.