Cheer.parse/3 leaves leaf commands warning about a missing run/2

[joshrotenberg]

Follow-up to #132.

A command tree consumed only through Cheer.parse/3 never needs run/2. The
compiler still warns:

warning: HexpmMcp.CLI is a leaf command (no subcommands) but does not implement run/2
  lib/hexpm_mcp/cli.ex:1: HexpmMcp.CLI (module)

run: 2 is in @optional_callbacks, so the behaviour already treats it as
optional; only the DSL compiler's leaf check disagrees.

This blocks the exact pattern parse/3 was added for. Projects that build with
mix compile --warnings-as-errors (which hexpm-mcp does, joshrotenberg/hexpm-mcp#68)
have to define a run/2 they never call in order to compile, which is the stub
#132 set out to remove.

Why it can't just be dropped

The warning is right for run/3 users: a leaf with no handler means dispatch
reaches a dead end at runtime. The compiler cannot tell which entry point a
consumer will use, so the intent has to be declared.

Suggestion

A DSL marker inside the command block:

command "hexpm_mcp" do
  parse_only()
  about "MCP server for hex.pm and hexdocs.pm"
  option :transport, type: :string, choices: ["stdio", "http"]
end

Suppresses the leaf warning, and could reasonably make Cheer.run/3 raise a
clear error for that command rather than failing obscurely.

use Cheer.Command, parse_only: true would work too, though the DSL form keeps
it next to the rest of the command's declaration.

Whatever the spelling, the cookbook entry from #133 should use it, since a
long-running server is precisely the case that has no run/2.