feat(inference): operator-tunable sampling knobs (top_k/min_p/repeat_penalty) for stage requests

GenerationConfig only carried temperature/top_p/max_tokens/stop/seed. Added nullable
topK/minP/repeatPenalty, serialized to the llama.cpp and OpenAI-compat request bodies
via @EncodeDefault(NEVER) so an unset knob is omitted (the model keeps its own default)
and behavior is unchanged unless the operator opts in.

Surfaced as a new [sampling] config section feeding the default stage GenerationConfig
(the main agentic loop) through TomlWorkflowLoader + ExecutionPlanCompiler; the former
hardcoded temperature=0.7/topP=1.0 stage defaults now come from config. Talkie
chat/narration keep their own generation settings.

Vikunja #46 (task 76) — sampling half.

infrastructure/inference/openai_compat/src/main/kotlin/com/correx/infrastructure/inference/openai/OpenAiApiModels.kt

@@ -2,6 +2,7 @@ package com.correx.infrastructure.inference.openai
 
 import com.correx.core.inference.ToolCallRequest
 import com.correx.core.inference.ToolDefinition
+import kotlinx.serialization.EncodeDefault
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
@@ -20,6 +21,11 @@ data class OpenAiChatCompletionRequest(
     @SerialName("max_tokens") val maxTokens: Int,
     @SerialName("stop") val stopSequences: List<String>? = null,
     val seed: Long? = null,
+    // Non-standard OpenAI params accepted by local backends (vLLM, llama.cpp --api). EncodeDefault.NEVER
+    // omits them when unset so a strict endpoint never sees an unknown key unless the operator opts in.
+    @EncodeDefault(EncodeDefault.Mode.NEVER) @SerialName("top_k") val topK: Int? = null,
+    @EncodeDefault(EncodeDefault.Mode.NEVER) @SerialName("min_p") val minP: Double? = null,
+    @EncodeDefault(EncodeDefault.Mode.NEVER) @SerialName("repeat_penalty") val repeatPenalty: Double? = null,
     val stream: Boolean = false,
     val tools: List<ToolDefinition>? = null,
 )

infrastructure/inference/openai_compat/src/main/kotlin/com/correx/infrastructure/inference/openai/OpenAiCompatInferenceProvider.kt

@@ -104,6 +104,9 @@ class OpenAiCompatInferenceProvider(
             maxTokens = request.generationConfig.maxTokens,
             stopSequences = request.generationConfig.stopSequences.ifEmpty { null },
             seed = request.generationConfig.seed,
+            topK = request.generationConfig.topK,
+            minP = request.generationConfig.minP,
+            repeatPenalty = request.generationConfig.repeatPenalty,
             stream = false,
             tools = tools,
         )