feat(lathe): add __lathe verify for generated CLI contract validation

[samzong]

What's changed?

• Add hidden __lathe verify --json command that validates generated CLI contract locally without network calls
• Check root help, catalog schema version, JSON marshalability, commands show parity, required flags/body inputs, and unauthenticated auth status behavior
• Add silentExitError in runtime so verify can emit JSON to stdout and exit non-zero without duplicating error output on stderr
• Document the verify step in README and CLI usage agent workflow

Why

• Agents need a fast, offline gate after building a generated CLI to confirm catalog/help/auth contracts before live API calls
• Verify failures should return structured JSON on stdout for machine inspection while keeping stderr clean

Verification

go test ./pkg/lathe/... ./pkg/runtime/... -count=1
go vet ./pkg/lathe/... ./pkg/runtime/...

Considered and deferred

• pkg/lathe/verify.go:54 [BOT-NIT]: --json flag is documented for agents but verify always emits JSON; flag exists for contract discoverability only.
• pkg/lathe/verify.go:171 [BOT-TASTE]: isJSONBody treats empty media type as JSON, matching OpenAPI default body semantics.