feat: add g0 config validate command

[kai-agent-free]

Summary

Adds g0 config validate [path] command that checks .g0.yaml for errors before running a scan.

Features

• Validates all known config keys: preset, min_score, min_grade, fail_on, exclude_rules, exclude_paths, rules_dir, severity_overrides, thresholds, analyzers, domain_weights, risk_accepted
• Reports unknown keys with "did you mean?" suggestions (e.g. strickt → strict)
• Type validation for all fields (numbers, strings, booleans, arrays, mappings)
• Warns on out-of-range values (e.g. min_score > 100)
• Validates risk_accepted entries for required rule/reason fields
• Exits 0 on valid config, 1 on errors
• Accepts optional path argument (file or directory)

Files

• src/cli/commands/config.ts — new config command with validate subcommand
• src/cli/index.ts — register config command
• tests/unit/config-validate.test.ts — 18 unit tests

Verification

• tsc --noEmit passes ✅
• All 18 tests pass ✅

Closes #85