Add Config::validate() called from load_or_init() after env overrides
are applied. This catches obviously invalid configuration values at
startup instead of allowing them to silently cause runtime failures.
Validated fields:
- gateway.host: must not be empty
- autonomy.max_actions_per_hour: must be > 0
- scheduler.max_concurrent: must be > 0
- scheduler.max_tasks: must be > 0
- model_routes[*]: hint, provider, model must not be empty
- embedding_routes[*]: hint, provider, model must not be empty
- proxy: delegates to existing ProxyConfig::validate()
Previously, ProxyConfig::validate() was only called during
apply_env_overrides() and only warned/disabled on failure. The new
Config::validate() runs it as a hard error after all overrides are
resolved, ensuring proxy misconfiguration is surfaced early.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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