feat: P1 tool improvements and P5 Agent Skills standard compliance

P1: Tool小改善

1. tailバイナリ判定 (TextFileDetector):
- file_search tailモードでtext/binary判定を追加
- 先頭4KBのみ読み、BOM確認(UTF-8/UTF-16 LE/BE)→UTF-8 strict byte検証
- NULバイトのみ判定不可(UTF-16でNULが正常出現するため)
- binaryファイルはToolValidationExceptionで拒否
- TextFileDetectorクラスへ判定ロジックを分離

2. file_edit rename機能:
- file_edit.operation=renameを追加(新規toolではない)
- new_path property追加、Write capability要求(source/destination双方)
- destination既存時は禁止(overwrite不可)
- cross-volume renameは不支持、Tool failureとして拒否
- FileRenameTransaction + RecoveryManager file.rename recovery追加
- crash recovery: destination hash == journal記録の場合のみ戻す

P5: Agent Skills open standard準拠

3. Skill形式をagentskills.io標準へ移行:
- legacy単一Markdown形式(skills/*.md)を廃止
- <skill-name>/SKILL.md標準directory形式のみ対応
- YAML frontmatter必須(name/description)、YamlDotNetで解析
- validation: name必須(1-64文字,lowercase英数字+hyphen,親dir名一致),
  description必須(1-1024文字), malformed YAML reject, duplicate fatal

4. Progressive Disclosure:
- Stage 1 Discovery: name+descriptionのみをsystem messageへ(name+desc一覧)
- Stage 3 Instructions: 選択Skillの本文のみ注入
- PromptComposer: 全文埋め込みを廃止、カタログ形式へ変更
- PromptContext: Skills→AvailableSkills+SelectedSkillsへ分割
- 未選択Skill本文はpromptへ入らない

5. Internal Tool境界維持:
- skill_read等のInternal Tool追加なし
- Internal Toolは6群(file_search/file_read/file_edit/process_run/task_update/dir)のまま
- Skill選択・本文ロードはAgent Runtime/Prompt Context Managementの責務

6. AgentSkill data model (Agent.Core/Skills/AgentSkill.cs):
- RootPath, Name, Description, Instructions, License?, Compatibility?, Metadata?, AllowedTools?
- SkillLoader(Agent.App)がYamlDotNetでfrontmatter解析→AgentSkill構築

設計判断:
- renameはfile_edit operation拡張(新規toolではない)
- rmdir空dirのみ制約は維持
- Skill load != permission grant維持
- allowed-toolsはSandbox permissionとして解釈しない
- YamlDotNetはAgent.App側のみ使用(Agent.Coreへ依存追加なし)

変更対象:
- TextFileDetector.cs: 新規、text/binary判定
- WindowsToolBackend.cs: tail binary判定統合、rename case追加
- InternalToolCatalog.cs: file_edit operation enum に rename 追加、new_path property
- InternalToolValidator.cs: rename時new_path必須検証
- TransactionalFileEditor.cs: Rename() + FileRenameTransaction
- RecoveryManager.cs: file.rename recovery
- AgentSkill.cs: 新規data model
- SkillLoader.cs: 全面書き換え、YamlDotNet frontmatter解析、validation
- PromptComposer.cs: Progressive Disclosure、AvailableSkills+SelectedSkills
- AgentRuntimeSettings.cs: Skills型変更
- AgentRuntime.cs: SelectedSkills=[]で初期化
- Program.cs: SkillLoader戻り値型対応
- Specifications.md: FS-07 rename、Skill節Agent Skills standard準拠
- Design.md: §7.1 tail binary判定、§7.3 rename、AgentSkill model
- ImplementationPlan.md: Prompt順序更新、Skill標準準拠

テスト:
- SecurityPolicyTests.cs: tail binary拒否/UTF-8/UTF-16 text許可、rename正常/dest既存拒否/許可外拒否
- InternalToolTests.cs: rename validationテスト追加
- ConfigurationTests.cs: SkillLoader標準形式/missing frontmatter/name mismatch/duplicate tests
- PromptAndRuntimeTests.cs: PromptContext型変更対応、skill catalog検証

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
- Internal Tool 6群固定: 確認済み

1 files changed, 95 insertions, 10 deletions

diff --git a/src/Agent.App/Configuration/SkillLoader.cs b/src/Agent.App/Configuration/SkillLoader.cs
index 6923c12..bac1559 100644
--- a/src/Agent.App/Configuration/SkillLoader.cs
+++ b/src/Agent.App/Configuration/SkillLoader.cs
@@ -1,24 +1,109 @@
+using System.Text.RegularExpressions;
+using Agent.Core.Skills;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.NamingConventions;
+
 namespace Agent.App.Configuration;
 
 public static class SkillLoader
 {
-    public static string[] Load(string directory)
+    private static readonly Regex NamePattern = new("^[a-z0-9]([a-z0-9-]*[a-z0-9])?$", RegexOptions.Compiled);
+
+    public static IReadOnlyList<AgentSkill> Load(string directory)
     {
-        var paths = new List<string>();
-        paths.AddRange(Directory.EnumerateFiles(directory, "*.md", SearchOption.TopDirectoryOnly));
+        if (!Directory.Exists(directory)) return [];
+
+        var skills = new List<AgentSkill>();
+        var names = new HashSet<string>(StringComparer.Ordinal);
 
         foreach (string child in Directory.EnumerateDirectories(directory, "*", SearchOption.TopDirectoryOnly))
         {
             var info = new DirectoryInfo(child);
             if ((info.Attributes & FileAttributes.ReparsePoint) != 0) continue;
-            string standardSkill = Path.Combine(info.FullName, "SKILL.md");
-            if (File.Exists(standardSkill)) paths.Add(standardSkill);
+
+            string skillMd = Path.Combine(info.FullName, "SKILL.md");
+            if (!File.Exists(skillMd)) continue;
+
+            string dirName = info.Name;
+            string content = File.ReadAllText(skillMd);
+            var (frontmatter, body) = SplitFrontmatter(content);
+
+            if (frontmatter is null)
+                throw new SkillLoadException($"Skill '{dirName}': SKILL.md is missing YAML frontmatter.");
+
+            SkillFrontmatter fm = ParseFrontmatter(frontmatter);
+
+            ValidateSkill(dirName, fm, names);
+
+            skills.Add(new AgentSkill(
+                RootPath: info.FullName,
+                Name: fm.Name,
+                Description: fm.Description,
+                Instructions: body.Trim(),
+                License: fm.License,
+                Compatibility: fm.Compatibility,
+                Metadata: fm.Metadata,
+                AllowedTools: fm.AllowedTools));
+        }
+
+        return skills;
+    }
+
+    private static void ValidateSkill(string dirName, SkillFrontmatter fm, HashSet<string> names)
+    {
+        if (string.IsNullOrWhiteSpace(fm.Name))
+            throw new SkillLoadException($"Skill '{dirName}': name is required.");
+        if (fm.Name.Length < 1 || fm.Name.Length > 64)
+            throw new SkillLoadException($"Skill '{dirName}': name must be 1-64 characters.");
+        if (!NamePattern.IsMatch(fm.Name))
+            throw new SkillLoadException($"Skill '{dirName}': name must be lowercase alphanumeric with hyphens.");
+        if (!fm.Name.Equals(dirName, StringComparison.Ordinal))
+            throw new SkillLoadException($"Skill '{dirName}': name '{fm.Name}' does not match directory name.");
+        if (string.IsNullOrWhiteSpace(fm.Description))
+            throw new SkillLoadException($"Skill '{dirName}': description is required.");
+        if (fm.Description.Length > 1024)
+            throw new SkillLoadException($"Skill '{dirName}': description must be 1-1024 characters.");
+        if (!names.Add(fm.Name))
+            throw new SkillLoadException($"Duplicate skill name '{fm.Name}'.");
+    }
+
+    private static (string? Frontmatter, string Body) SplitFrontmatter(string content)
+    {
+        if (!content.StartsWith("---", StringComparison.Ordinal)) return (null, content);
+        int end = content.IndexOf("\n---", 3, StringComparison.Ordinal);
+        if (end < 0) return (null, content);
+        string frontmatter = content[3..end].Trim();
+        int bodyStart = end + 4;
+        if (bodyStart < content.Length && content[bodyStart] == '\r') bodyStart++;
+        if (bodyStart < content.Length && content[bodyStart] == '\n') bodyStart++;
+        string body = bodyStart < content.Length ? content[bodyStart..] : string.Empty;
+        return (frontmatter, body);
+    }
+
+    private static SkillFrontmatter ParseFrontmatter(string yaml)
+    {
+        try
+        {
+            var deserializer = new DeserializerBuilder()
+                .WithNamingConvention(UnderscoredNamingConvention.Instance)
+                .Build();
+            return deserializer.Deserialize<SkillFrontmatter>(yaml);
+        }
+        catch (Exception ex)
+        {
+            throw new SkillLoadException($"Malformed YAML frontmatter: {ex.Message}");
         }
+    }
 
-        return paths
-            .Distinct(StringComparer.OrdinalIgnoreCase)
-            .Order(StringComparer.OrdinalIgnoreCase)
-            .Select(File.ReadAllText)
-            .ToArray();
+    private sealed class SkillFrontmatter
+    {
+        public string Name { get; set; } = "";
+        public string Description { get; set; } = "";
+        public string? License { get; set; }
+        public string? Compatibility { get; set; }
+        public Dictionary<string, string>? Metadata { get; set; }
+        public string? AllowedTools { get; set; }
     }
 }
+
+public sealed class SkillLoadException(string message) : Exception(message);