Add discovery-helper tests (env var override, PATH fallback, missing
binary) and command tests that point M2PACK_BINARY at a Python stub
script echoing canned JSON per subcommand. Cover success paths,
non-zero exit, non-JSON output, JSON-array output, and missing binary.
Extend the MCP schema mirror test to cover all 12 tools and add
dispatch tests for the new m2pack_* argv translation.
Pytest suite with a tiny_client fixture, an ephemeral Ed25519 keypair
fixture, and a threaded HTTPServer helper. Exercises cli dispatch,
inspect (including excluded-path handling), build-manifest and sign
against the real m2dev-client scripts, diff-remote via a local server,
and the full release publish composite against a local rsync target.
