feat(KB-617): add diff viewer tab to task detail modal

- Add modifiedFiles and baseCommitSha tracking during task execution
- Create GET /tasks/:id/diff API endpoint for file list and patches
- Build TaskChangesTab component with expandable file diffs
- Integrate Changes tab into TaskDetailModal for in-progress, in-review, and done tasks
- Add database columns and types for diff tracking
- Update executor to capture modified files during agent sessions

packages/dashboard/app/styles.css

@@ -11812,3 +11812,134 @@ html .column.drag-over * {
 [data-theme="light"] .gm-load-more:hover {
   background: rgba(0, 0, 0, 0.03);
 }
+
+/* ── Task Changes Tab Styles ─────────────────────────────────────────────── */
+
+.task-changes-tab {
+  padding: 16px;
+}
+
+.changes-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 16px;
+}
+
+.changes-header h4 {
+  margin: 0;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-size: 14px;
+  font-weight: 500;
+}
+
+.changes-file-list {
+  border: 1px solid var(--border, #30363d);
+  border-radius: 8px;
+  overflow: hidden;
+}
+
+.changes-file-item {
+  border-bottom: 1px solid var(--border, #30363d);
+}
+
+.changes-file-item:last-child {
+  border-bottom: none;
+}
+
+.changes-file-item.expanded {
+  background: var(--bg-secondary, #161b22);
+}
+
+.changes-file-header {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 10px 12px;
+  background: none;
+  border: none;
+  width: 100%;
+  text-align: left;
+  cursor: pointer;
+  color: var(--text-primary, #c9d1d9);
+  font-size: 13px;
+  transition: background 0.15s;
+}
+
+.changes-file-header:hover {
+  background: var(--bg-hover, #1f242c);
+}
+
+.changes-file-toggle {
+  display: flex;
+  align-items: center;
+  color: var(--text-secondary, #8b949e);
+  flex-shrink: 0;
+}
+
+.changes-file-status {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 20px;
+  height: 20px;
+  border-radius: 4px;
+  font-size: 11px;
+  font-weight: 600;
+  flex-shrink: 0;
+}
+
+.changes-file-path {
+  flex: 1;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
+}
+
+.changes-file-stat {
+  color: var(--text-secondary, #8b949e);
+  font-size: 11px;
+  flex-shrink: 0;
+  margin-left: 8px;
+}
+
+.changes-file-content {
+  border-top: 1px solid var(--border, #30363d);
+  background: var(--bg-primary, #0d1117);
+}
+
+.changes-diff-patch {
+  margin: 0;
+  padding: 12px;
+  font-size: 12px;
+  line-height: 1.5;
+  overflow-x: auto;
+  white-space: pre;
+  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
+  color: var(--text-primary, #c9d1d9);
+}
+
+.changes-diff-patch code {
+  background: none;
+  padding: 0;
+}
+
+/* Syntax highlighting for diff */
+.changes-diff-patch .diff-add,
+.changes-diff-patch [data-prefix="+"] {
+  color: #3fb950;
+}
+
+.changes-diff-patch .diff-del,
+.changes-diff-patch [data-prefix="-"] {
+  color: #f85149;
+}
+
+.changes-diff-patch .diff-hunk,
+.changes-diff-patch [data-prefix="@@"] {
+  color: #58a6ff;
+}