feat: port spec show validate plan apply into Python gr2 CLI

[laynepenney]

Python gr2 is now the active UX authority. We need the declarative workspace flow to live under the same Python CLI surface as workspace/lane/review/hooks.

Scope:

• add gr2 spec show
• add gr2 spec validate
• add gr2 plan
• add gr2 apply
• keep command names/JSON shapes as the Python contract
• do not preserve Rust-only UX assumptions unless they still fit the Python-first model

Why:

• current gr2 authority is split across Python and Rust
• this is the main blocker to treating Python gr2 as one coherent product surface
• Rust can be ported/reimplemented later behind the proven interface

Acceptance:

• normal workspace flow can stay entirely inside Python gr2 for spec->plan->apply
• output is usable by both humans and agents
• no dependence on the shipped Rust gr2 binary