fix: execute carve commands from explicit argv

[shaug]

TL;DR

Replace implicit shell parsing in carve-changesets with validated explicit argv while preserving intentional shell workflows.

Summary

• Execute validation and database-comparison commands as explicit argv without implicit shell parsing.
• Reject legacy string flags and unknown command representations before lifecycle mutation, with precise migration guidance.
• Document explicit shell migrations for pipelines, redirects, quoting, environment expansion, and compound commands.
• Add focused unit and integration coverage for the executor, plan schema, lifecycle boundaries, and every legacy CLI surface.

Validation

• just format
• just lint
• just test

Tickets

Refs #71