feat: add repository workspace as default project view

- Create git file utilities (list_tree, get_file_content, list_branches, commit_file)
- Add file browsing API endpoints (list, content, branches, update)
- Create RepoWorkspace page with sidebar + main content layout
- Add FileTree component with directory navigation
- Add FileViewer component for viewing file contents
- Update project list to link to workspace
- Add workspace CSS styles
- Update router with workspace route

Quality gates: ruff ✓, mypy ✓, typecheck ✓, build ✓

apps/web/src/styles.css

@@ -694,3 +694,151 @@ a {
     top: 1rem;
   }
 }
+
+/* Repository Workspace */
+.repo-workspace {
+  display: flex;
+  flex-direction: column;
+  height: calc(100vh - 60px);
+  overflow: hidden;
+}
+
+.workspace-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 1rem;
+  border-bottom: 1px solid var(--border);
+  background: var(--panel);
+}
+
+.workspace-title {
+  display: flex;
+  align-items: center;
+  gap: 1rem;
+}
+
+.workspace-title h1 {
+  margin: 0;
+  font-size: 1.25rem;
+}
+
+.repo-name {
+  color: var(--muted);
+  font-size: 0.875rem;
+}
+
+.workspace-layout {
+  display: flex;
+  flex: 1;
+  overflow: hidden;
+}
+
+.workspace-sidebar {
+  width: 280px;
+  min-width: 280px;
+  border-right: 1px solid var(--border);
+  background: var(--panel);
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+}
+
+.sidebar-section {
+  padding: 1rem;
+  border-bottom: 1px solid var(--border);
+}
+
+.sidebar-section label {
+  margin: 0;
+}
+
+.workspace-main {
+  flex: 1;
+  overflow: auto;
+  padding: 1rem;
+  background: var(--bg);
+}
+
+/* File Tree */
+.file-tree {
+  flex: 1;
+  overflow: auto;
+  padding: 0.5rem;
+}
+
+.tree-entry {
+  display: block;
+  width: 100%;
+  padding: 0.375rem 0.5rem;
+  border: none;
+  background: none;
+  color: var(--ink);
+  text-align: left;
+  cursor: pointer;
+  border-radius: 4px;
+  font-size: 0.875rem;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.tree-entry:hover {
+  background: var(--bg);
+}
+
+.tree-directory {
+  font-weight: 500;
+}
+
+.tree-up {
+  color: var(--muted);
+  font-style: italic;
+}
+
+/* File Viewer */
+.file-viewer {
+  background: var(--panel);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  overflow: hidden;
+}
+
+.file-viewer-header {
+  padding: 0.75rem 1rem;
+  border-bottom: 1px solid var(--border);
+  background: var(--bg);
+}
+
+.file-breadcrumbs {
+  font-size: 0.875rem;
+  font-family: monospace;
+}
+
+.breadcrumb-sep {
+  color: var(--muted);
+  margin: 0 0.25rem;
+}
+
+.file-content {
+  padding: 1rem;
+  overflow: auto;
+  max-height: calc(100vh - 200px);
+}
+
+.file-content pre {
+  margin: 0;
+  font-family: "IBM Plex Mono", monospace;
+  font-size: 0.875rem;
+  line-height: 1.5;
+  white-space: pre-wrap;
+  word-wrap: break-word;
+}
+
+.file-viewer-empty {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  height: 100%;
+  min-height: 300px;
+}