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

wwwroot/index.html

@@ -144,6 +144,10 @@
         <div id="agentFilesSection" hidden>
           <button type="button" id="agentFilesBtn" class="add-cmd-btn"><i class="fa fa-folder"></i> Dateien verwalten</button>
         </div>
+        <div id="agentSkillsSection" hidden>
+          <label>Skills</label>
+          <div class="skill-list" id="agentSkillList"></div>
+        </div>
         <label>Benutzerdefinierte Befehle</label>
         <div class="custom-commands" id="agentCommandsList"></div>
         <button type="button" id="agentAddCommandBtn" class="add-cmd-btn">+ Befehl hinzufügen</button>