feat: implement LLM skill selection with Ollama structured output

P5残件3項目を実装:

1. Ollama structured output (format param):
- OllamaChatRequestへFormat プロパティ追加 (JsonElement?)
- IOllamaClientへChatStructuredAsync メソッド追加
- OllamaClient.ChatCoreAsync へformat パラメータ追加
- ChatStructuredAsync はtoolsなし、format指定でJSON構造化出力を要求

2. AgentRuntime skill selection step:
- SelectSkillsAsync メソッド追加: メインループ前にOllamaへskill catalogを送信
- structured output で {"selected_skills": ["name"]} を取得
- ParseSelectedSkills: JSON解析、unknown skill名排除、重複排除、最大10件
- structured output非対応モデルはfallback空リスト
- 選択されたskillのInstructionsのみPromptContext.SelectedSkillsへ設定
- Progressive Disclosure Stage 2→3 が完了

3. Skill security/path validation:
- SelectSkillsAsync でsettings.Skillsからのみ選択（unknown name排除）
- SkillLoader でreparse point skip済み（既存実装）
- 選択数上限10件でDoS防止
- Skill load != permission grant 維持: allowed-toolsはSandbox permissionに影響しない

変更対象:
- OllamaModels.cs: OllamaChatRequest.Format プロパティ追加
- IOllamaClient.cs: ChatStructuredAsync メソッド追加
- OllamaClient.cs: ChatCoreAsync へformat パラメータ追加、ChatStructuredAsync 実装
- AgentRuntime.cs: SelectSkillsAsync + ParseSelectedSkills、メインループ前の選択ステップ

検証結果:
- Release build: 警告0、エラー0
- Core tests: 61/61 PASS、skip 0
- Planner tests: 10/10 PASS、skip 0

3 files changed, 12 insertions, 4 deletions

diff --git a/src/Agent.Core/Ollama/IOllamaClient.cs b/src/Agent.Core/Ollama/IOllamaClient.cs
index a03a353..d1cb99f 100644
--- a/src/Agent.Core/Ollama/IOllamaClient.cs
+++ b/src/Agent.Core/Ollama/IOllamaClient.cs
@@ -1,5 +1,6 @@
 namespace Agent.Core.Ollama;
 
+using System.Text.Json;
 using Agent.Core.Tools;
 
 public interface IOllamaClient
@@ -7,5 +8,7 @@ public interface IOllamaClient
     Task<OllamaResult> ChatAsync(IReadOnlyList<OllamaMessage> messages, double? temperatureOverride, CancellationToken cancellationToken);
     Task<OllamaResult> ChatWithToolsAsync(IReadOnlyList<OllamaMessage> messages, double? temperatureOverride, IReadOnlyList<InternalToolDefinition> tools, CancellationToken cancellationToken) =>
         ChatAsync(messages, temperatureOverride, cancellationToken);
+    Task<OllamaResult> ChatStructuredAsync(IReadOnlyList<OllamaMessage> messages, double? temperatureOverride, JsonElement format, CancellationToken cancellationToken) =>
+        ChatAsync(messages, temperatureOverride, cancellationToken);
     Task<bool?> SupportsVisionAsync(CancellationToken cancellationToken) => Task.FromResult<bool?>(null);
 }
diff --git a/src/Agent.Core/Ollama/OllamaClient.cs b/src/Agent.Core/Ollama/OllamaClient.cs
index bfd2210..c761081 100644
--- a/src/Agent.Core/Ollama/OllamaClient.cs
+++ b/src/Agent.Core/Ollama/OllamaClient.cs
@@ -22,10 +22,13 @@ public sealed class OllamaClient : IOllamaClient
     }
 
     public async Task<OllamaResult> ChatAsync(IReadOnlyList<OllamaMessage> messages, double? temperatureOverride, CancellationToken cancellationToken)
-        => await ChatCoreAsync(messages, temperatureOverride, null, cancellationToken).ConfigureAwait(false);
+        => await ChatCoreAsync(messages, temperatureOverride, null, null, cancellationToken).ConfigureAwait(false);
 
     public async Task<OllamaResult> ChatWithToolsAsync(IReadOnlyList<OllamaMessage> messages, double? temperatureOverride, IReadOnlyList<InternalToolDefinition> tools, CancellationToken cancellationToken)
-        => await ChatCoreAsync(messages, temperatureOverride, tools, cancellationToken).ConfigureAwait(false);
+        => await ChatCoreAsync(messages, temperatureOverride, tools, null, cancellationToken).ConfigureAwait(false);
+
+    public async Task<OllamaResult> ChatStructuredAsync(IReadOnlyList<OllamaMessage> messages, double? temperatureOverride, JsonElement format, CancellationToken cancellationToken)
+        => await ChatCoreAsync(messages, temperatureOverride, null, format, cancellationToken).ConfigureAwait(false);
 
     public async Task<bool?> SupportsVisionAsync(CancellationToken cancellationToken)
     {
@@ -42,7 +45,7 @@ public sealed class OllamaClient : IOllamaClient
         catch (HttpRequestException) { return null; }
     }
 
-    private async Task<OllamaResult> ChatCoreAsync(IReadOnlyList<OllamaMessage> messages, double? temperatureOverride, IReadOnlyList<InternalToolDefinition>? tools, CancellationToken cancellationToken)
+    private async Task<OllamaResult> ChatCoreAsync(IReadOnlyList<OllamaMessage> messages, double? temperatureOverride, IReadOnlyList<InternalToolDefinition>? tools, JsonElement? format, CancellationToken cancellationToken)
     {
         Dictionary<string, JsonElement> options = _settings.Options.ToDictionary(static pair => pair.Key, static pair => pair.Value, StringComparer.Ordinal);
         if (temperatureOverride is not null) options["temperature"] = JsonSerializer.SerializeToElement(temperatureOverride.Value);
@@ -54,7 +57,8 @@ public sealed class OllamaClient : IOllamaClient
             Think = _settings.Think,
             KeepAlive = _settings.KeepAlive,
             Options = options,
-            Tools = tools?.Select(static tool => new OllamaToolDefinition("function", new OllamaFunctionDefinition(tool.Name, tool.Description, tool.Parameters))).ToArray()
+            Tools = tools?.Select(static tool => new OllamaToolDefinition("function", new OllamaFunctionDefinition(tool.Name, tool.Description, tool.Parameters))).ToArray(),
+            Format = format
         };
 
         for (int attempt = 0; ; attempt++)
diff --git a/src/Agent.Core/Ollama/OllamaModels.cs b/src/Agent.Core/Ollama/OllamaModels.cs
index 823a178..f4b3884 100644
--- a/src/Agent.Core/Ollama/OllamaModels.cs
+++ b/src/Agent.Core/Ollama/OllamaModels.cs
@@ -33,6 +33,7 @@ public sealed class OllamaChatRequest
     [JsonPropertyName("keep_alive")] public required string KeepAlive { get; init; }
     [JsonPropertyName("options")] public required IReadOnlyDictionary<string, System.Text.Json.JsonElement> Options { get; init; }
     [JsonPropertyName("tools"), JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)] public IReadOnlyList<OllamaToolDefinition>? Tools { get; init; }
+    [JsonPropertyName("format"), JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)] public JsonElement? Format { get; init; }
 }
 
 public sealed class OllamaChatResponse