Implement Skills feature: filesystem-based skills with agent assignment

- Add SkillService: scans data/skills/ directory, parses _meta.json and SKILL.md frontmatter
- Add SkillEndpoint: REST API for listing skills and managing agent-skill assignments
- Add agent_skills DB table for agent-to-skill mapping (only relation in DB, skills live on filesystem)
- Extend SystemPromptBuilder: inject assigned skills' SKILL.md content into system prompt
- Extend ChatEndpoint + SessionEndpoint: load agent skills and pass to prompt builder
- Frontend: skill checkboxes in agent form, API functions for skill management
- Add form.css styling for skill list items

Services/SkillService.cs

@@ -0,0 +1,138 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text.Json;
+
+namespace Gott.Services;
+
+public class SkillService
+{
+    private readonly string _skillsDir;
+
+    public string SkillsDir => _skillsDir;
+
+    public SkillService(string dataDir)
+    {
+        _skillsDir = Path.Combine(dataDir, "skills");
+        Directory.CreateDirectory(_skillsDir);
+    }
+
+    /// <summary>
+    /// Scannt den Skills-Ordner und gibt alle verfügbaren Skills zurück.
+    /// </summary>
+    public List<SkillInfo> GetAllSkills()
+    {
+        var skills = new List<SkillInfo>();
+        if (!Directory.Exists(_skillsDir)) return skills;
+
+        foreach (var dir in Directory.GetDirectories(_skillsDir))
+        {
+            var skill = LoadSkill(dir);
+            if (skill != null) skills.Add(skill);
+        }
+
+        return skills.OrderBy(s => s.Name).ToList();
+    }
+
+    /// <summary>
+    /// Lädt einen einzelnen Skill anhand seines Slugs.
+    /// </summary>
+    public SkillInfo GetSkill(string slug)
+    {
+        var dir = Path.Combine(_skillsDir, slug);
+        if (!Directory.Exists(dir)) return null;
+        return LoadSkill(dir);
+    }
+
+    /// <summary>
+    /// Liest die SKILL.md eines Skills. Gibt null zurück wenn nicht vorhanden.
+    /// </summary>
+    public string GetSkillPrompt(string slug)
+    {
+        var skillMd = Path.Combine(_skillsDir, slug, "SKILL.md");
+        if (!File.Exists(skillMd)) return null;
+        return File.ReadAllText(skillMd);
+    }
+
+    private SkillInfo LoadSkill(string dir)
+    {
+        var slug = Path.GetFileName(dir);
+        var metaFile = Path.Combine(dir, "_meta.json");
+        var skillMd = Path.Combine(dir, "SKILL.md");
+
+        var info = new SkillInfo
+        {
+            Slug = slug,
+            Path = Path.GetFullPath(dir),
+            HasSkillMd = File.Exists(skillMd)
+        };
+
+        // Name/Description aus SKILL.md Frontmatter lesen
+        if (File.Exists(skillMd))
+        {
+            var content = File.ReadAllText(skillMd);
+            var frontmatter = ParseFrontmatter(content);
+            if (frontmatter.TryGetValue("name", out var name)) info.Name = name;
+            if (frontmatter.TryGetValue("description", out var desc)) info.Description = desc;
+            if (frontmatter.TryGetValue("version", out var ver)) info.Version = ver;
+        }
+
+        // _meta.json als Fallback/Ergänzung
+        if (File.Exists(metaFile))
+        {
+            try
+            {
+                var json = File.ReadAllText(metaFile);
+                using var doc = JsonDocument.Parse(json);
+                var root = doc.RootElement;
+                if (string.IsNullOrEmpty(info.Name) && root.TryGetProperty("name", out var n))
+                    info.Name = n.GetString();
+                if (string.IsNullOrEmpty(info.Version) && root.TryGetProperty("version", out var v))
+                    info.Version = v.GetString();
+                if (root.TryGetProperty("slug", out var s) && !string.IsNullOrEmpty(s.GetString()))
+                    info.Slug = s.GetString();
+            }
+            catch { /* Fehler in _meta.json ignorieren */ }
+        }
+
+        // Fallback: Name = Slug
+        if (string.IsNullOrEmpty(info.Name)) info.Name = slug;
+
+        return info;
+    }
+
+    /// <summary>
+    /// Parst einen einfachen YAML-Frontmatter-Block (--- ... ---) aus einer Markdown-Datei.
+    /// </summary>
+    private Dictionary<string, string> ParseFrontmatter(string content)
+    {
+        var result = new Dictionary<string, string>();
+        if (!content.StartsWith("---")) return result;
+
+        var end = content.IndexOf("---", 3, StringComparison.Ordinal);
+        if (end < 0) return result;
+
+        var frontmatter = content.Substring(3, end - 3).Trim();
+        foreach (var line in frontmatter.Split('\n'))
+        {
+            var colon = line.IndexOf(':');
+            if (colon < 0) continue;
+            var key = line.Substring(0, colon).Trim();
+            var value = line.Substring(colon + 1).Trim();
+            result[key] = value;
+        }
+
+        return result;
+    }
+}
+
+public class SkillInfo
+{
+    public string Slug { get; set; }
+    public string Name { get; set; }
+    public string Description { get; set; }
+    public string Version { get; set; }
+    public string Path { get; set; }
+    public bool HasSkillMd { get; set; }
+}