Refactor LLM config into top-level settings

Replace the monolithic agents config with a cleaner structure:
- llm: holds provider credentials and default model settings
- probo-agent: LLM overrides for the probo agent
- evidence-describer: worker config (interval, stale-after,
  max-concurrency) alongside LLM overrides

This makes worker tuning configurable via YAML and env vars
instead of being hardcoded in Go, and separates provider
credentials from per-consumer model settings.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/probod/llm_config.go

@@ -0,0 +1,59 @@
+// Copyright (c) 2025-2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package probod
+
+type (
+	// LLMProviderConfig holds authentication and connection settings for an
+	// LLM provider (e.g. OpenAI, Anthropic).
+	LLMProviderConfig struct {
+		Type   string `json:"type"`    // "openai", "anthropic", "bedrock"
+		APIKey string `json:"api-key"` // for OpenAI and Anthropic
+	}
+
+	// LLMConfig holds model parameters for a single LLM consumer. Provider
+	// references one of the keys in LLMSettings.Providers.
+	LLMConfig struct {
+		Provider    string   `json:"provider"` // key into LLMSettings.Providers
+		ModelName   string   `json:"model-name"`
+		Temperature *float64 `json:"temperature"`
+		MaxTokens   *int     `json:"max-tokens"`
+	}
+
+	// LLMSettings groups LLM provider credentials and default model
+	// settings. Defaults is used as a fallback when a consumer-specific
+	// field is zero-valued.
+	LLMSettings struct {
+		Providers map[string]LLMProviderConfig `json:"providers"`
+		Defaults  LLMConfig                    `json:"defaults"`
+	}
+)
+
+// ResolveLLMConfig returns a fully populated LLMConfig by filling in
+// zero-valued fields from the defaults.
+func (s *LLMSettings) ResolveLLMConfig(cfg LLMConfig) LLMConfig {
+	if cfg.Provider == "" {
+		cfg.Provider = s.Defaults.Provider
+	}
+	if cfg.ModelName == "" {
+		cfg.ModelName = s.Defaults.ModelName
+	}
+	if cfg.Temperature == nil {
+		cfg.Temperature = s.Defaults.Temperature
+	}
+	if cfg.MaxTokens == nil {
+		cfg.MaxTokens = s.Defaults.MaxTokens
+	}
+	return cfg
+}