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/css/form.css

@@ -123,6 +123,63 @@
   font-size: var(--font-size);
 }
 
+/* ── Skills ── */
+
+.skill-list {
+  display: flex;
+  flex-direction: column;
+  gap: var(--badge-spacing);
+}
+
+.skill-item {
+  display: flex;
+  align-items: center;
+  gap: var(--badge-spacing);
+  cursor: pointer;
+  padding: var(--badge-spacing) var(--box-spacing);
+  background: var(--f2-bg);
+  border: var(--border-width) solid var(--f2-border);
+  border-radius: var(--border-radius);
+  margin-top: 0;
+}
+
+.skill-item:hover {
+  border-color: var(--accent);
+}
+
+.skill-item input[type="checkbox"] {
+  width: 18px;
+  height: 18px;
+  accent-color: var(--accent);
+  cursor: pointer;
+}
+
+.skill-name {
+  font-weight: 600;
+}
+
+.skill-version {
+  opacity: 0.5;
+  font-weight: 400;
+}
+
+.skill-desc {
+  opacity: 0.6;
+  font-size: var(--font-size);
+  margin-left: auto;
+}
+
+.skill-empty {
+  opacity: 0.5;
+  padding: var(--box-spacing);
+}
+
+.skill-empty code {
+  background: var(--f2-bg);
+  padding: 2px 6px;
+  border-radius: var(--border-radius);
+}
+
 .danger-btn {
   color: var(--dangerous);
   border-color: var(--dangerous);