Add browser extension and shell completions (Milestones 12-13)

M12: Browser Extension
- Chrome/Edge Manifest V3 extension
- Popup UI for saving pages
- Context menu integration (save page/selection/link)
- Background service worker
- Content script for extraction

M13: Shell Completions
- Bash, Zsh, Fish, PowerShell completions
- Dynamic node ID completion
- Dynamic tag completion
- Dynamic graph completion
- Auto-install command (--install)

extension/popup/popup.css

@@ -0,0 +1,136 @@
+* {
+  box-sizing: border-box;
+  margin: 0;
+  padding: 0;
+}
+
+body {
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+  font-size: 14px;
+  width: 320px;
+  padding: 16px;
+  background: #1a1a2e;
+  color: #eee;
+}
+
+.container {
+  display: flex;
+  flex-direction: column;
+  gap: 12px;
+}
+
+h1 {
+  font-size: 18px;
+  font-weight: 600;
+  color: #4fc3f7;
+  margin-bottom: 8px;
+}
+
+.status {
+  padding: 8px;
+  border-radius: 4px;
+  font-size: 12px;
+  display: none;
+}
+
+.status.success {
+  display: block;
+  background: #1b5e20;
+  color: #a5d6a7;
+}
+
+.status.error {
+  display: block;
+  background: #b71c1c;
+  color: #ef9a9a;
+}
+
+.field {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+}
+
+.field.checkbox {
+  flex-direction: row;
+  align-items: center;
+}
+
+.field.checkbox label {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  cursor: pointer;
+}
+
+label {
+  font-size: 12px;
+  color: #aaa;
+  font-weight: 500;
+}
+
+input[type="text"],
+select {
+  padding: 8px 12px;
+  border: 1px solid #333;
+  border-radius: 4px;
+  background: #0f0f23;
+  color: #eee;
+  font-size: 14px;
+}
+
+input[type="text"]:focus,
+select:focus {
+  outline: none;
+  border-color: #4fc3f7;
+}
+
+select {
+  cursor: pointer;
+}
+
+input[type="checkbox"] {
+  width: 16px;
+  height: 16px;
+  cursor: pointer;
+}
+
+.actions {
+  display: flex;
+  gap: 8px;
+  margin-top: 8px;
+}
+
+button {
+  flex: 1;
+  padding: 10px 16px;
+  border: none;
+  border-radius: 4px;
+  font-size: 14px;
+  font-weight: 500;
+  cursor: pointer;
+  transition: background 0.2s;
+}
+
+button.primary {
+  background: #4fc3f7;
+  color: #000;
+}
+
+button.primary:hover {
+  background: #29b6f6;
+}
+
+button:not(.primary) {
+  background: #333;
+  color: #eee;
+}
+
+button:not(.primary):hover {
+  background: #444;
+}
+
+button:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}