feat: Agent configuration form - Create and edit agents from UI (COM-18)

[Kaiohz]

Summary

• Add complete form-based agent configuration UI, allowing agents to be created and edited entirely from the interface (in addition to existing YAML upload)
• Form serializes to YAML client-side and uploads via existing multipart API (no backend changes needed)
• Zod validation schemas for all AgentConfig types with react-hook-form integration

Changes

New Components

• AgentConfigForm: Full form with accordion sections (General, System Prompt, Tools & Middleware, Backend, HITL, Memory & Skills, MCP Servers, Subagents, Response Format)
• StringListEditor: Reusable dynamic string list (tools, skills, memory, args)
• KeyValueEditor: Reusable key-value map editor (headers, env, HITL rules)
• McpServerEditor: Nested form for MCP servers with conditional STDIO/HTTP fields
• SubAgentEditor: Nested form for subagents with tools/skills editors
• shadcn/ui components: Dialog, Form, Input, Select, Switch, Tabs, Accordion, Textarea, ScrollArea, Button, Label, Separator, Badge

Modified Components

• CreateAgentDialog: Added Tabs (Form / Upload YAML), form tab is default
• AgentConfigViewer: Added Edit mode with pre-filled form + Export YAML download button
• AgentsPage: Changed button text from "Create Agent (YAML)" to "Create Agent"

New Domain Layer

• agentConfigSchema.ts: Zod validation schemas for AgentConfig, BackendConfig, HITLConfig, McpServerConfig, SubAgentConfig
• yaml.ts: serializeAgentConfig(), parseAgentConfig(), agentConfigToYamlFile() utilities using js-yaml

Tests

• 273/274 tests passing (1 pre-existing chat API failure unrelated to this PR)
• New test files: agentConfigSchema.test.ts, yaml.test.ts, AgentConfigForm.test.tsx, StringListEditor.test.tsx, KeyValueEditor.test.tsx, McpServerEditor.test.tsx, SubAgentEditor.test.tsx
• Updated existing tests: CreateAgentDialog.test.tsx, AgentConfigViewer.test.tsx, AgentsPage.test.tsx

Dependencies

• Added js-yaml + @types/js-yaml for YAML serialization

Jira

COM-18