feat(core): port v2 command invocation - #34849

Summary

• add v2 session command invocation that expands commands into normal durable prompt admission
• expose MCP prompts as command-shaped entries and resolve them at invocation time
• make /review a normal command for now and defer subtask/backgrounding semantics
• add the v2 session.command API and regenerate generated clients

Follow-up

• Subtask/backgrounding decision issue: v2: decide command execution and mention semantics #34847
• MCP prompt optimistic UI issue: v2: add optimistic UI while MCP prompt commands resolve #34860

Verification

• Not run per request. Earlier typecheck attempts were stopped before completion.