refactor: deprecate camelCase aliases in targets.yaml schema

[christso]

Problem

The targets.yaml schema accepts both snake_case and camelCase for several fields:

• grader_target / graderTarget (not used — only judge_target has camelCase alias)
• provider_batching / providerBatching
• subagent_mode_allowed / subagentModeAllowed
• timeout_seconds / timeoutSeconds
• log_dir / logDir / log_directory / logDirectory
• log_format / logFormat
• system_prompt / systemPrompt
• api_key / apiKey
• base_url / baseUrl
• etc.

YAML configs should use snake_case exclusively (matching the wire format convention in AGENTS.md). camelCase is for TypeScript internals only.

Proposal

1. Emit deprecation warnings when camelCase variants are used in targets.yaml
2. In a future major version, remove camelCase acceptance from the schema
3. New fields (like use_target) should only support snake_case

This is a minor, non-breaking refactor — existing configs continue working but users see warnings guiding them to snake_case.