feat: add git integration with file browsing and diff viewing

Add comprehensive git support across the stack:
- CLI: register git RPC handlers (status, diff numstat, diff file)
- Server: create git routes with proper session path resolution
- Web: add file browser, diff viewer, and git status visualization
- Add FileIcon component and git parser utilities
- Add TanStack Router routes for /files and /file pages
- Add git-themed CSS variables for light and dark modes

web/src/components/SessionHeader.tsx

@@ -16,9 +16,30 @@ function getSessionTitle(session: Session): string {
     return session.id.slice(0, 8)
 }
 
+function FilesIcon(props: { className?: string }) {
+    return (
+        <svg
+            xmlns="http://www.w3.org/2000/svg"
+            width="18"
+            height="18"
+            viewBox="0 0 24 24"
+            fill="none"
+            stroke="currentColor"
+            strokeWidth="2"
+            strokeLinecap="round"
+            strokeLinejoin="round"
+            className={props.className}
+        >
+            <path d="M14 2H7a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h10a2 2 0 0 0 2-2V8z" />
+            <path d="M14 2v6h6" />
+        </svg>
+    )
+}
+
 export function SessionHeader(props: {
     session: Session
     onBack: () => void
+    onViewFiles?: () => void
 }) {
     const title = useMemo(() => getSessionTitle(props.session), [props.session])
 
@@ -60,6 +81,17 @@ export function SessionHeader(props: {
                         {props.session.metadata?.path ?? props.session.id}
                     </div>
                 </div>
+
+                {props.onViewFiles ? (
+                    <button
+                        type="button"
+                        onClick={props.onViewFiles}
+                        className="flex h-8 w-8 items-center justify-center rounded-full text-[var(--app-hint)] transition-colors hover:bg-[var(--app-secondary-bg)] hover:text-[var(--app-fg)]"
+                        title="Files"
+                    >
+                        <FilesIcon />
+                    </button>
+                ) : null}
             </div>
         </div>
     )