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, 0 deletions

diff --git a/src/Agent.Windows/Recovery/TransactionalFileEditor.cs b/src/Agent.Windows/Recovery/TransactionalFileEditor.cs
index 805a069..25c949d 100644
--- a/src/Agent.Windows/Recovery/TransactionalFileEditor.cs
+++ b/src/Agent.Windows/Recovery/TransactionalFileEditor.cs
@@ -93,6 +93,26 @@ public sealed class TransactionalFileEditor(TaskResourceJournal journal, string
         return new(journal, entryId, canonical);
     }
 
+    public FileRenameTransaction Rename(string sourcePath, string destinationPath)
+    {
+        string source = CanonicalPath.Resolve(sourcePath);
+        string destination = CanonicalPath.Resolve(destinationPath, requireExisting: false);
+        if (!File.Exists(source)) throw new FileNotFoundException($"Source file does not exist: '{source}'.");
+        if (File.Exists(destination)) throw new ToolValidationException($"Destination already exists: '{destination}'. Rename overwrite is not allowed.");
+        if (!source.StartsWith(System.IO.Path.GetPathRoot(source)!, StringComparison.OrdinalIgnoreCase) ||
+            !destination.StartsWith(System.IO.Path.GetPathRoot(source)!, StringComparison.OrdinalIgnoreCase))
+            throw new ToolValidationException("Cross-volume rename is not supported.");
+        byte[] content = File.ReadAllBytes(source);
+        string sourceHash = Convert.ToHexString(SHA256.HashData(content));
+        Guid entryId = journal.Prepare("file.rename", source, new Dictionary<string, string>
+        {
+            ["destination_path"] = destination, ["source_sha256"] = sourceHash
+        });
+        File.Move(source, destination);
+        journal.MarkApplied(entryId);
+        return new(journal, entryId, source, destination, sourceHash);
+    }
+
     private static string NormalizeHash(string hash) => hash.Replace("-", string.Empty, StringComparison.Ordinal).Trim().ToUpperInvariant();
 }
 
@@ -144,6 +164,27 @@ public sealed class DirectoryRmdirTransaction(TaskResourceJournal journal, Guid
     }
 }
 
+public sealed class FileRenameTransaction(TaskResourceJournal journal, Guid entryId, string sourcePath, string destinationPath, string sourceHash)
+{
+    private bool _finished;
+    public void Commit() { if (_finished) return; journal.MarkCleaned(entryId); _finished = true; }
+    public void Rollback()
+    {
+        if (_finished) return;
+        if (!File.Exists(destinationPath)) throw new InvalidOperationException("Rollback refused because the destination file is missing.");
+        if (!Hash(destinationPath).Equals(sourceHash, StringComparison.Ordinal))
+            throw new InvalidDataException("Rollback refused because the destination file hash does not match the original source hash.");
+        File.Move(destinationPath, sourcePath);
+        journal.MarkCleaned(entryId);
+        _finished = true;
+    }
+    private static string Hash(string target)
+    {
+        using FileStream stream = File.OpenRead(target);
+        return Convert.ToHexString(SHA256.HashData(stream));
+    }
+}
+
 public sealed class FileEditTransaction(
     TaskResourceJournal journal, Guid entryId, string path, string originalHash, string replacementHash,
     string backupPath, string originalBase64, bool originallyExisted)