feat: add validate_package_request RPC and fix validate_prompt dispatch in cograw

[jeanmachuca]

Add package management validation RPC and fix missing validate_prompt types/dispatch.

• handleValidatePackageRequest handler with 6 compiled-in validation rules:
  rate limiting (5 ops/5min), critical package check, has_raw_model guard,
  disk space check, registry allowlist, read-only passthrough
• Add ValidatePackageRequestParams/ValidatePackageRequestResult types
• Rate limiter state (packageRateMu, packageRateCount, packageRateStart)
• criticalPackages blocklist (cognitiveosd, raw-model, base-os)
• Add missing ValidatePromptParams/ValidatePromptResult types
• Add missing validate_prompt dispatch entry (was defined but unreachable)
• AGENTS.md: add validate_package_request to RPC method table