feat(providers): OpenAI-compatible provider types as runtime data (providers.yaml)

Replace the five copy-paste OpenAI-compatible provider structs (moonshot,
moonshot_code, deepseek, zai, lm_studio) with one DeclaredProvider engine
driven by a providers.yaml catalog loaded at boot from the cwd — edit and
restart, no rebuild. The YAML carries identity, endpoints, per-model JSON
field mapping, id-glob enrichment rules and the reasoning knob (effort /
thinking request kinds); capability_flags keep vision and future input
modalities declarative. Anthropic, Ollama, OpenAI and OpenRouter stay
native (different wire protocols or bespoke parsing) and register
alongside; colliding declared ids are skipped. The shipped catalog is
validated by a unit test.

crates/skald-core/src/llm/providers/mod.rs

@@ -1,11 +1,8 @@
 pub mod anthropic;
-pub mod deepseek;
-pub mod lm_studio;
-pub mod moonshot;
+pub mod declared;
 pub mod ollama;
 pub mod openai;
 pub mod openrouter;
-pub mod zai;
 
 // Re-export so existing code that uses `providers::ServiceType` / `providers::RemoteLlmModelInfo` keeps working.
 pub use crate::provider::ServiceType;