refactor: split validate-options.ts into focused validator modules (#3479)

* Initial plan

* refactor: split validate-options.ts into focused validator modules

Split the 450-line validateOptions function into four focused modules
under src/commands/validators/:

- log-and-limits.ts: log level, model multipliers, resource limits
- network-options.ts: Docker host, domain resolution, network config
- agent-options.ts: env vars, volume mounts, SSL Bump URL patterns
- config-assembly.ts: config assembly + post-config validations

validate-options.ts is now a thin ~33-line orchestrator that calls the
sub-validators in sequence. Public API is unchanged.

All 1991 existing tests continue to pass.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot