Agents: CRUD, DB table, session binding, replace model picker

- agents table with name, avatar, description, model, persona, instructions, soft-delete
- AgentEndpoint (GET/POST/PUT/DELETE /api/agents)
- sessions + chains now reference agent_id
- ChatEndpoint resolves agent from session (no agent = no chat)
- system prompt built from agent persona + instructions
- frontend: agent editor with avatar upload, agent cards on dashboard
- session: agent dropdown replaces model picker, choice persisted

wwwroot/css/main.css

@@ -129,3 +129,83 @@ body {
 .placeholder-icon {
   font-size: 4em;
 }
+
+/* ── Agent Form ── */
+
+.agent-form {
+  padding: calc(var(--s4) * 2);
+  overflow-y: auto;
+  display: flex;
+  flex-direction: column;
+  gap: var(--s3);
+  max-width: 600px;
+}
+
+.agent-form label {
+  font-weight: bold;
+}
+
+.agent-form textarea {
+  resize: vertical;
+}
+
+.agent-avatar-section {
+  display: flex;
+  align-items: center;
+  gap: var(--s4);
+}
+
+.agent-avatar-preview {
+  width: 64px;
+  height: 64px;
+  background: var(--f2-bg);
+  border: var(--border-width) solid var(--f2-border);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 2em;
+  overflow: hidden;
+  flex-shrink: 0;
+}
+
+.agent-avatar-preview img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+}
+
+.agent-form-actions {
+  display: flex;
+  gap: var(--s4);
+  margin-top: var(--s4);
+}
+
+.danger-btn {
+  color: #ff6b6b;
+  border-color: #ff6b6b;
+}
+
+.danger-btn:hover {
+  background: #ff6b6b;
+  color: var(--f1-bg);
+}
+
+/* ── Agent Card Avatar ── */
+
+.agent-card-avatar {
+  width: 40px;
+  height: 40px;
+  background: var(--f2-bg);
+  border: var(--border-width) solid var(--f2-border);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 1.2em;
+  overflow: hidden;
+}
+
+.agent-card-avatar img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+}