[Agentic DX] g: low-to-high lift CLI contract and module layout roadmap

[tony]

Context

Review focused on src/g/__init__.py (parser + runtime bundled), packaging metadata, and CLI behavior.

Goal

Keep g minimal while making it more automatable and standards-aligned.

Low-lift ideas

• Move CLI runtime from g.__init__ into dedicated modules (cli.py, runner.py).
• Add --json mode to report detected VCS, executed command, and exit code.
• Document exit code semantics for no-repo / command-failure paths.

Medium-lift ideas

• Add --dry-run to show what would execute without side effects.
• Add pluggable VCS marker detection (beyond .git/.hg/.svn) behind a strategy abstraction.
• Add contract tests for parser and machine-output behavior.

High-lift ideas

• Add g capabilities --json for environment and detector introspection.
• Publish a tiny machine-readable command contract in docs.

Done when

• Human and machine output are both first-class.
• Runtime structure is modular.
• Behavior is explicit and testable.