feat: Add conversational AI assistant panel for project codebase Q&A

Implement a slide-in chat panel that allows users to ask questions about
their codebase using Claude Opus 4.5 with read-only access to project files.

Backend changes:
- Add SQLAlchemy models for conversation persistence (assistant_database.py)
- Create AssistantChatSession with read-only Claude SDK client
- Add WebSocket endpoint for real-time chat streaming
- Include read-only MCP tools: feature_get_stats, feature_get_next, etc.

Frontend changes:
- Add floating action button (bottom-right) to toggle panel
- Create slide-in panel component (400px width)
- Implement WebSocket hook with reconnection logic
- Add keyboard shortcut 'A' to toggle assistant

Key features:
- Read-only access: Only Read, Glob, Grep, WebFetch, WebSearch tools
- Persistent history: Conversations saved to SQLite per project
- Real-time streaming: Text chunks streamed as Claude generates response
- Tool visibility: Shows when assistant is using tools to explore code

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/components/AssistantPanel.tsx

@@ -0,0 +1,79 @@
+/**
+ * Assistant Panel Component
+ *
+ * Slide-in panel container for the project assistant chat.
+ * Slides in from the right side of the screen.
+ */
+
+import { X, Bot } from 'lucide-react'
+import { AssistantChat } from './AssistantChat'
+
+interface AssistantPanelProps {
+  projectName: string
+  isOpen: boolean
+  onClose: () => void
+}
+
+export function AssistantPanel({ projectName, isOpen, onClose }: AssistantPanelProps) {
+  return (
+    <>
+      {/* Backdrop - click to close */}
+      {isOpen && (
+        <div
+          className="fixed inset-0 bg-black/20 z-40 transition-opacity duration-300"
+          onClick={onClose}
+          aria-hidden="true"
+        />
+      )}
+
+      {/* Panel */}
+      <div
+        className={`
+          fixed right-0 top-0 bottom-0 z-50
+          w-[400px] max-w-[90vw]
+          bg-white
+          border-l-4 border-[var(--color-neo-border)]
+          shadow-[-8px_0_0px_rgba(0,0,0,1)]
+          transform transition-transform duration-300 ease-out
+          flex flex-col
+          ${isOpen ? 'translate-x-0' : 'translate-x-full'}
+        `}
+        role="dialog"
+        aria-label="Project Assistant"
+        aria-hidden={!isOpen}
+      >
+        {/* Header */}
+        <div className="flex items-center justify-between px-4 py-3 border-b-3 border-[var(--color-neo-border)] bg-[var(--color-neo-progress)]">
+          <div className="flex items-center gap-2">
+            <div className="bg-white border-2 border-[var(--color-neo-border)] p-1.5 shadow-[2px_2px_0px_rgba(0,0,0,1)]">
+              <Bot size={18} />
+            </div>
+            <div>
+              <h2 className="font-display font-bold text-white">Project Assistant</h2>
+              <p className="text-xs text-white/80 font-mono">{projectName}</p>
+            </div>
+          </div>
+          <button
+            onClick={onClose}
+            className="
+              neo-btn neo-btn-ghost
+              p-2
+              bg-white/20 border-white/40
+              hover:bg-white/30
+              text-white
+            "
+            title="Close Assistant (Press A)"
+            aria-label="Close Assistant"
+          >
+            <X size={18} />
+          </button>
+        </div>
+
+        {/* Chat area */}
+        <div className="flex-1 overflow-hidden">
+          {isOpen && <AssistantChat projectName={projectName} />}
+        </div>
+      </div>
+    </>
+  )
+}