feat(workspace): no double resume, server-picked tabs, terminal a11y

- A resume of a session a live pane already holds is refused (409
  ESESSIONLIVE with error.run_id) and the client opens that pane, pasting
  any typed message there; catalog tools refuse resumeSessionId
  (EUNSUPPORTED) instead of silently starting fresh.
- POST /api/lanes/:id/start takes slot: "auto": the server picks the free
  tab under the lane lock (components/lanes/startSession.ts on the client,
  kept out of WorkspaceHome so the sidebar does not pull react-markdown
  into the main bundle).
- Unsaved whole-file edits are guarded on every exit (Back, another file,
  tab switch, close, lane/session change, beforeunload).
- Closing an AI tab asks first; hiding a pane has its own icon. Tabs and
  pane headers show the session name, a state dot, rename by double-click.
- Terminal: Ctrl+M toggles Tab-moves-focus (no keyboard trap), Cmd+F /
  Ctrl+Shift+F find bar, clickable links (noopener), a write-only OSC 52
  clipboard, a Reconnecting overlay, debounced resizes, optional screen
  reader mode. Alt+1-4 / Alt+\ / Alt+W focus, split and hide panes.
- Changes panel: draggable splitter, wrapping git bar, history paging,
  disarming discard/undo/abort, inline diff comments pasted only into a
  visible pane; drag MIME guard on sidebar rows; IME-safe Enter; Show
  more in the sidebar and Recents; idle dots readable with sr-only text.
- Adds @xterm/addon-clipboard, @xterm/addon-search, @xterm/addon-web-links.

client/package-lock.json

@@ -11,7 +11,10 @@
         "@fontsource/inter": "^5.2.8",
         "@fontsource/jetbrains-mono": "^5.2.8",
         "@xterm/addon-canvas": "^0.7.0",
+        "@xterm/addon-clipboard": "^0.1.0",
         "@xterm/addon-fit": "^0.10.0",
+        "@xterm/addon-search": "^0.15.0",
+        "@xterm/addon-web-links": "^0.11.0",
         "@xterm/xterm": "^5.5.0",
         "d3": "^7.9.0",
         "i18next": "^26.0.8",
@@ -2136,6 +2139,18 @@
         "@xterm/xterm": "^5.0.0"
       }
     },
+    "node_modules/@xterm/addon-clipboard": {
+      "version": "0.1.0",
+      "resolved": "https://registry.npmjs.org/@xterm/addon-clipboard/-/addon-clipboard-0.1.0.tgz",
+      "integrity": "sha512-zdoM7p53T5sv/HbRTyp4hY0kKmEQ3MZvAvEtiXqNIHc/JdpqwByCtsTaQF5DX2n4hYdXRPO4P/eOS0QEhX1nPw==",
+      "license": "MIT",
+      "dependencies": {
+        "js-base64": "^3.7.5"
+      },
+      "peerDependencies": {
+        "@xterm/xterm": "^5.4.0"
+      }
+    },
     "node_modules/@xterm/addon-fit": {
       "version": "0.10.0",
       "resolved": "https://registry.npmjs.org/@xterm/addon-fit/-/addon-fit-0.10.0.tgz",
@@ -2145,6 +2160,24 @@
         "@xterm/xterm": "^5.0.0"
       }
     },
+    "node_modules/@xterm/addon-search": {
+      "version": "0.15.0",
+      "resolved": "https://registry.npmjs.org/@xterm/addon-search/-/addon-search-0.15.0.tgz",
+      "integrity": "sha512-ZBZKLQ+EuKE83CqCmSSz5y1tx+aNOCUaA7dm6emgOX+8J9H1FWXZyrKfzjwzV+V14TV3xToz1goIeRhXBS5qjg==",
+      "license": "MIT",
+      "peerDependencies": {
+        "@xterm/xterm": "^5.0.0"
+      }
+    },
+    "node_modules/@xterm/addon-web-links": {
+      "version": "0.11.0",
+      "resolved": "https://registry.npmjs.org/@xterm/addon-web-links/-/addon-web-links-0.11.0.tgz",
+      "integrity": "sha512-nIHQ38pQI+a5kXnRaTgwqSHnX7KE6+4SVoceompgHL26unAxdfP6IPqUTSYPQgSwM56hsElfoNrrW5V7BUED/Q==",
+      "license": "MIT",
+      "peerDependencies": {
+        "@xterm/xterm": "^5.0.0"
+      }
+    },
     "node_modules/@xterm/xterm": {
       "version": "5.5.0",
       "resolved": "https://registry.npmjs.org/@xterm/xterm/-/xterm-5.5.0.tgz",
@@ -3711,6 +3744,12 @@
         "jiti": "bin/jiti.js"
       }
     },
+    "node_modules/js-base64": {
+      "version": "3.9.4",
+      "resolved": "https://registry.npmjs.org/js-base64/-/js-base64-3.9.4.tgz",
+      "integrity": "sha512-PtOMXpEGuP0RRiRXsjzHzl44dMHxSu2CPvAhinupR1tBa88me+1DPqsobl7eupn5UukjLkln1ZnAOAOXOrYG0Q==",
+      "license": "BSD-3-Clause"
+    },
     "node_modules/js-tokens": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/js-tokens/-/js-tokens-4.0.0.tgz",