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/js/api.js

@@ -144,6 +144,35 @@ async function apiGitStatus(sessionId) {
   return resp.json();
 }
 
+// ── Skills API ──
+
+async function apiGetSkills() {
+  const resp = await fetch('/api/skills');
+  return resp.json();
+}
+
+async function apiGetAgentSkills(agentId) {
+  const resp = await fetch(`/api/agents/${agentId}/skills`);
+  return resp.json();
+}
+
+async function apiSetAgentSkills(agentId, skills) {
+  const resp = await fetch(`/api/agents/${agentId}/skills`, {
+    method: 'PUT',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ skills })
+  });
+  return resp.json();
+}
+
+async function apiAddAgentSkill(agentId, slug) {
+  return fetch(`/api/agents/${agentId}/skills/${slug}`, { method: 'POST' });
+}
+
+async function apiRemoveAgentSkill(agentId, slug) {
+  return fetch(`/api/agents/${agentId}/skills/${slug}`, { method: 'DELETE' });
+}
+
 async function apiBackupDb() {
   const resp = await fetch('/api/backup', { method: 'POST' });
   if (!resp.ok) throw new Error('Backup fehlgeschlagen');