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, 41 insertions, 4 deletions

diff --git a/tests/Agent.Core.Tests/ConfigurationTests.cs b/tests/Agent.Core.Tests/ConfigurationTests.cs
index 2c52b0c..79c746d 100644
--- a/tests/Agent.Core.Tests/ConfigurationTests.cs
+++ b/tests/Agent.Core.Tests/ConfigurationTests.cs
@@ -1,5 +1,6 @@
 using Agent.App.Cli;
 using Agent.App.Configuration;
+using Agent.Core.Skills;
 
 namespace Agent.Core.Tests;
 
@@ -43,16 +44,52 @@ public sealed class ConfigurationTests
     }
 
     [Fact]
-    public void SkillLoader_supports_legacy_markdown_and_standard_skill_directories()
+    public void SkillLoader_loads_standard_skill_directories_only()
     {
         using var directory = new TestDirectory();
+        // Legacy single markdown — should be ignored (not loaded).
         directory.File("skills/legacy.md", "legacy skill");
-        directory.File("skills/python-via-uv/SKILL.md", "standard skill");
+        // Standard skill directory with frontmatter.
+        directory.File("skills/python-via-uv/SKILL.md", "---\nname: python-via-uv\ndescription: Python execution skill.\n---\n# Python via uv\nInstructions here.");
+        // Non-SKILL.md files in skill directory should not be loaded as skills.
         directory.File("skills/python-via-uv/reference.md", "must not be loaded as a skill");
 
-        string[] skills = SkillLoader.Load(Path.Combine(directory.Path, "skills"));
+        var skills = SkillLoader.Load(Path.Combine(directory.Path, "skills"));
 
-        Assert.Equal(["legacy skill", "standard skill"], skills);
+        Assert.Single(skills);
+        Assert.Equal("python-via-uv", skills[0].Name);
+        Assert.Equal("Python execution skill.", skills[0].Description);
+        Assert.Contains("Instructions here.", skills[0].Instructions);
+    }
+
+    [Fact]
+    public void SkillLoader_rejects_missing_frontmatter()
+    {
+        using var directory = new TestDirectory();
+        directory.File("skills/no-frontmatter/SKILL.md", "# No frontmatter\nJust body.");
+
+        Assert.Throws<SkillLoadException>(() => SkillLoader.Load(Path.Combine(directory.Path, "skills")));
+    }
+
+    [Fact]
+    public void SkillLoader_rejects_name_mismatch()
+    {
+        using var directory = new TestDirectory();
+        directory.File("skills/wrong-name/SKILL.md", "---\nname: different-name\ndescription: A skill.\n---\nBody.");
+
+        Assert.Throws<SkillLoadException>(() => SkillLoader.Load(Path.Combine(directory.Path, "skills")));
+    }
+
+    [Fact]
+    public void SkillLoader_rejects_duplicate_names()
+    {
+        using var directory = new TestDirectory();
+        directory.File("skills/dup-skill/SKILL.md", "---\nname: dup-skill\ndescription: First.\n---\nBody1.");
+        // Create a second directory with the same skill name (via symlink or junction is not needed;
+        // just create another dir that also has name: dup-skill in frontmatter).
+        directory.File("skills/dup-skill-copy/SKILL.md", "---\nname: dup-skill\ndescription: Second.\n---\nBody2.");
+
+        Assert.Throws<SkillLoadException>(() => SkillLoader.Load(Path.Combine(directory.Path, "skills")));
     }
 
     private static string CreateValidConfiguration(TestDirectory directory)