[refactor] Fail-fast Validation via PreRunE and MarkFlagRequired

[kangeunchan]

Enhancement Description

Make CLI validation fail-fast by moving checks to PreRunE/MarkFlagRequired and centralizing shared validators.

Background

Commands currently validate inputs inside RunE after expensive work (service init, config I/O, network checks). This causes multi-second delays before reporting invalid input and proliferates duplicated validation logic.

Scope

Move validations from RunE to PreRunE across commands

Files: cmd/devnet-builder/commands/, cmd/dvb/

Use declarative required flags

Files: cmd/devnet-builder/commands/, cmd/dvb/

• Replace manual “empty string” checks with MarkFlagRequired where applicable

Centralize shared validation helpers

Files: internal/cmd/validation/** (new package)

• Mode validation
• Validator count validation
• Network validation
• Daemon requirement checks (promote to parent PersistentPreRunE where possible)

Non-Goals

• Changing command behavior beyond validation timing
• Introducing new flags or breaking existing CLI interface
• Large refactors of application service initialization

Risks and Open Questions

• Some validations currently depend on config load; ensure cheap validation still possible (or fast-path load)
• Cobra hook ordering must be consistent for subcommands and persistent hooks
• Error message consistency needs alignment across commands

Validation Plan

Unit and Integration Checks

• go test ./... for command packages and validation package
• Unit tests for internal/cmd/validation helpers
• Ensure commands fail before service initialization for invalid inputs (test with fakes)

End-to-End Checks

• Manual smoke: run invalid-flag combinations and confirm errors return immediately
• Smoke deploy/upgrade/provision still work with correct inputs

Evidence Required in Issue Updates

• Timing evidence (command logs or simple benchmarks) showing <100ms failure on invalid input
• Diff of removed manual validation patterns (examples)
• Example error outputs showing consistent messages

Acceptance Criteria

• Invalid inputs fail before service initialization
• Required flags use MarkFlagRequired where appropriate
• Cross-flag validation is in PreRunE
• Manual validation occurrences reduced significantly (target: < 50)
• No command initializes services before failing validation

Deliverables

• PR migrating validations to PreRunE and introducing shared validation package
• Documentation update (if needed) for validation behavior and error messages

[kangeunchan]

pull request: #106