Replace stringly-typed provider selection with ProviderKind enum

[bug-ops]

Part of #391

Problem

LlmConfig.provider is a String matched against "ollama", "claude", etc. in multiple match arms. Typos fall silently to error branches with no compile-time exhaustiveness.

File: crates/zeph-core/src/config/types.rs line 60

Proposed Solution

Define enum ProviderKind { Ollama, Claude, OpenAi, Candle, Compatible(String), Orchestrator, Router } with #[serde(rename_all = "lowercase")].

Priority: P2 | Effort: Small