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

1 files changed, 66 insertions, 1 deletions

diff --git a/src/Agent.Core/Runtime/AgentRuntime.cs b/src/Agent.Core/Runtime/AgentRuntime.cs
index 228ac41..f6bb483 100644
--- a/src/Agent.Core/Runtime/AgentRuntime.cs
+++ b/src/Agent.Core/Runtime/AgentRuntime.cs
@@ -1,3 +1,4 @@
+using System.Text.Json;
 using Agent.Core.Ollama;
 using Agent.Core.Prompts;
 using Agent.Core.Sessions;
@@ -28,12 +29,16 @@ public sealed class AgentRuntime(TaskRegistry registry, IOllamaClient ollama, Ag
             await _tools.BeginTaskAsync(taskId, cancellationToken).ConfigureAwait(false);
             if (task.Images.Count > 0 && await ollama.SupportsVisionAsync(cancellationToken).ConfigureAwait(false) is false)
                 throw new InvalidOperationException(settings.VisionUnsupportedMessage);
+
+            // Progressive Disclosure Stage 2: LLM Skill Selection.
+            IReadOnlyList<AgentSkill> selectedSkills = await SelectSkillsAsync(task.Request, cancellationToken).ConfigureAwait(false);
+
             var messages = PromptComposer.Compose(new PromptContext(
                 FixedPrompt.Load(),
                 settings.UserSystemPrompt,
                 settings.Locale,
                 settings.Skills,
-                [],  // SelectedSkills: populated after LLM skill selection (Progressive Disclosure Stage 2→3).
+                selectedSkills,  // Stage 3: selected skill instructions injected by PromptComposer.
                 settings.SandboxCapabilitySummary,
                 "Current task: " + task.Request,
                 string.Empty,
@@ -134,6 +139,66 @@ public sealed class AgentRuntime(TaskRegistry registry, IOllamaClient ollama, Ag
         }
     }
 
+    private async Task<IReadOnlyList<AgentSkill>> SelectSkillsAsync(string request, CancellationToken cancellationToken)
+    {
+        if (settings.Skills.Count == 0) return [];
+
+        // Build a minimal prompt with skill catalog for the LLM to choose from.
+        string catalog = string.Join("\n", settings.Skills.Select(static s => $"- {s.Name}: {s.Description}"));
+        var selectionMessages = new List<OllamaMessage>
+        {
+            new("system", $"Select skills relevant to the user request. Available skills:\n{catalog}\n\nReturn a JSON object with a 'selected_skills' array containing skill names. Select only relevant skills. If none are relevant, return an empty array."),
+            new("user", request)
+        };
+
+        // JSON Schema for structured output.
+        string schema = """{"type":"object","properties":{"selected_skills":{"type":"array","items":{"type":"string"}}},"required":["selected_skills"]}""";
+        using JsonDocument formatDoc = JsonDocument.Parse(schema);
+
+        OllamaResult result;
+        try
+        {
+            result = await ollama.ChatStructuredAsync(selectionMessages, 0.0, formatDoc.RootElement.Clone(), cancellationToken).ConfigureAwait(false);
+        }
+        catch
+        {
+            // If structured output fails (e.g., model doesn't support it), fall back to no skills.
+            return [];
+        }
+
+        return ParseSelectedSkills(result.Content);
+    }
+
+    private IReadOnlyList<AgentSkill> ParseSelectedSkills(string content)
+    {
+        try
+        {
+            using JsonDocument doc = JsonDocument.Parse(content);
+            if (!doc.RootElement.TryGetProperty("selected_skills", out JsonElement skillsElement)) return [];
+            var selectedNames = new HashSet<string>(StringComparer.Ordinal);
+            foreach (JsonElement item in skillsElement.EnumerateArray())
+            {
+                string? name = item.GetString();
+                if (!string.IsNullOrWhiteSpace(name)) selectedNames.Add(name);
+            }
+            // Validate: only known skills, no duplicates, max 10.
+            var valid = new List<AgentSkill>();
+            foreach (AgentSkill skill in settings.Skills)
+            {
+                if (selectedNames.Contains(skill.Name))
+                {
+                    valid.Add(skill);
+                    if (valid.Count >= 10) break;
+                }
+            }
+            return valid;
+        }
+        catch
+        {
+            return [];
+        }
+    }
+
     private async Task<string?> EndTaskSafelyAsync(Guid taskId, bool success, CancellationToken cancellationToken)
     {
         try