Extract ProviderFactory from main.rs to zeph-llm

[bug-ops]

Part of #391

Problem

Provider construction logic is duplicated 3 times in src/main.rs:

• create_provider (lines 775-927)
• create_named_provider (lines 930-1012)
• build_orchestrator (lines 1214-1340)

Every new provider backend requires changes in 3 places.

Proposed Solution

Introduce ProviderFactory::from_config(name: &str, config: &Config) -> Result<AnyProvider> in zeph-llm. Router and Orchestrator call it for sub-providers.

Priority: P0 | Effort: Medium