dev: initial implementation of notebook-snapshot extension

Manual-commit notebook snapshot extension (JupyterLab 4):

Backend
- Storage adapter layer (Local + S3 via boto3, pip extra)
  - Dumb put/get/list/delete contract; key validation
  - S3Storage: lazy boto3 import, S3ConnectionError with friendly
    messages + connectivity check at startup
- SnapshotStore
  - Strip code outputs/execution_count, MD5 on cleaned JSON
  - Gzipped envelope (id/timestamp/name/description/hash/size/notebook)
  - Append-only manifest.json (rebuildable from version files)
  - SnapshotUnchangedError when new hash matches most recent version
- REST API (commit / list / content) under /snapshot/ namespace
- traitlets config (storage_type, local_root, s3_* + S3_* env fallback)

Frontend
- snapshot:commit command (toolbar button + command palette)
- Dialog for name + description
- SnapshotPanel (left sidebar timeline) + DiffWidget (main area)
- Cell diff: id-match first, LCS fallback; jsdiff for line diff
- Restore via model.fromJSON() + context.save() (no refresh)
- All AGENTS.md hard conventions enforced

Tests: 33 backend pytest passing (storage + store + routes)

Docs: AGENTS.md, design.md, README.md synced with implementation.

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

style/base.css

@@ -0,0 +1,206 @@
+/*
+    See the JupyterLab Developer Guide for useful CSS Patterns:
+
+    https://jupyterlab.readthedocs.io/en/stable/developer/css.html
+*/
+
+/* snapshot extension */
+
+.jp-snapshot-panel {
+  padding: 8px;
+  overflow-y: auto;
+  height: 100%;
+  box-sizing: border-box;
+}
+
+.jp-snapshot-panel-empty {
+  color: var(--jp-ui-font-color2, #555);
+  padding: 12px;
+  text-align: center;
+}
+
+.jp-snapshot-panel-hint {
+  font-size: 0.85em;
+  margin-top: 8px;
+  color: var(--jp-ui-font-color2, #888);
+}
+
+.jp-snapshot-panel-list {
+  list-style: none;
+  margin: 0;
+  padding: 0;
+}
+
+.jp-snapshot-panel-item {
+  border: 1px solid var(--jp-border-color2, #ddd);
+  border-radius: 4px;
+  padding: 8px;
+  margin-bottom: 8px;
+  background: var(--jp-layout-color1, #fff);
+}
+
+.jp-snapshot-panel-item-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  gap: 8px;
+}
+
+.jp-snapshot-panel-name {
+  font-weight: 600;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.jp-snapshot-panel-size {
+  font-size: 0.8em;
+  color: var(--jp-ui-font-color2, #888);
+  white-space: nowrap;
+}
+
+.jp-snapshot-panel-desc {
+  font-size: 0.9em;
+  color: var(--jp-ui-font-color2, #555);
+  margin: 4px 0;
+  white-space: pre-wrap;
+}
+
+.jp-snapshot-panel-time {
+  font-size: 0.8em;
+  color: var(--jp-ui-font-color2, #888);
+  margin-bottom: 6px;
+}
+
+.jp-snapshot-panel-actions {
+  display: flex;
+  gap: 4px;
+}
+
+.jp-snapshot-panel-button {
+  flex: 1;
+  padding: 4px 8px;
+  font-size: 0.85em;
+  border: 1px solid var(--jp-border-color2, #ccc);
+  border-radius: 3px;
+  background: var(--jp-layout-color2, #f5f5f5);
+  cursor: pointer;
+}
+
+.jp-snapshot-panel-button:hover {
+  background: var(--jp-layout-color3, #e8e8e8);
+}
+
+/* commit dialog */
+
+.jp-snapshot-commit-dialog {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  padding: 8px;
+  min-width: 320px;
+}
+
+.jp-snapshot-commit-dialog label {
+  font-weight: 600;
+  font-size: 0.9em;
+}
+
+.jp-snapshot-commit-dialog-name,
+.jp-snapshot-commit-dialog-desc {
+  font-family: inherit;
+  font-size: 1em;
+  padding: 4px 6px;
+  border: 1px solid var(--jp-border-color2, #ccc);
+  border-radius: 3px;
+  box-sizing: border-box;
+  width: 100%;
+}
+
+.jp-snapshot-commit-dialog-desc {
+  resize: vertical;
+}
+
+/* diff widget */
+
+.jp-snapshot-diff-widget {
+  padding: 8px;
+  overflow-y: auto;
+  height: 100%;
+  box-sizing: border-box;
+}
+
+.jp-snapshot-diff-container {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+}
+
+.jp-snapshot-diff-row {
+  border: 1px solid var(--jp-border-color2, #ddd);
+  border-radius: 4px;
+  padding: 6px;
+  background: var(--jp-layout-color1, #fff);
+}
+
+.jp-snapshot-diff-added {
+  background: var(--jp-diff-added-color1, #e6ffed);
+  border-left: 3px solid #2cbe4e;
+}
+
+.jp-snapshot-diff-deleted {
+  background: var(--jp-diff-deleted-color1, #ffeef0);
+  border-left: 3px solid #cb2431;
+}
+
+.jp-snapshot-diff-modified {
+  background: var(--jp-diff-modified-color1, #fff5b1);
+  border-left: 3px solid #dfb700;
+}
+
+.jp-snapshot-diff-unchanged {
+  background: var(--jp-layout-color1, #fff);
+  border-left: 3px solid var(--jp-border-color2, #ddd);
+}
+
+.jp-snapshot-diff-badge {
+  display: inline-block;
+  font-size: 0.8em;
+  font-weight: 600;
+  padding: 1px 6px;
+  border-radius: 3px;
+  margin-right: 6px;
+  background: var(--jp-layout-color2, #eee);
+}
+
+.jp-snapshot-diff-cell-type {
+  display: inline-block;
+  font-size: 0.8em;
+  color: var(--jp-ui-font-color2, #888);
+}
+
+.jp-snapshot-diff-pre {
+  font-family: var(--jp-code-font-family, monospace);
+  font-size: 0.9em;
+  margin: 6px 0 0 0;
+  white-space: pre-wrap;
+  word-break: break-all;
+}
+
+.jp-snapshot-diff-line-added {
+  background: #ccffd8;
+}
+
+.jp-snapshot-diff-line-removed {
+  background: #ffd7d5;
+}
+
+.jp-snapshot-diff-line-context {
+  color: var(--jp-ui-font-color2, #555);
+}
+
+.jp-snapshot-diff-empty {
+  color: var(--jp-ui-font-color2, #888);
+  text-align: center;
+  padding: 16px;
+}

style/index.css

@@ -0,0 +1 @@
+@import url('base.css');

style/index.js

@@ -0,0 +1 @@
+import './base.css';