feat: Rails-style DX — claw new, claw g, claw model

[mostlydev]

Summary

Add a coherent generator language to the CLI so new users have a memorable, discoverable onramp.

• claw new <pod-name> — create a new pod (named alias for claw init, plus --template)
• claw g agent/contract/skill — additive generators (rename/alias of claw agent add + new stubs)
• claw model list/set/failover — CLI interface for model config without hand-editing Clawfiles
• Better claw help command grouping
• Bundled starter templates (quickstart, multi-agent, trading-pod, rollcall)

Full design in docs/plans/2026-03-27-rails-style-dx.md.

Nothing architecturally new — surfaces and formalizes what already exists. No breaking changes.