feat(workspace): AI tool per session, one place to start a session, ended sessions open like Claude Desktop

- A session picks its own AI tool: `tool` on POST /api/lanes/:id/start
  (a catalog id, known + enabled, else 400 EBADTOOL/EDISABLED); a resume
  keeps the tool its session ran on (`sessions.tool`). The workspace's
  `lanes.tool` is now only the default.
- The workspace home's prompt box is the one place a session starts. Its
  fields run in order: AI → AI Router (Native or a profile) → Model (the
  tool's own list, or the router's models) → Permission (Claude mode /
  Codex sandbox+approval / catalog bypass) → Effort.
- The pane's run form (RunSetup) is gone: New run, a single pane's exit and
  an empty split pane's New session all go to the home. RunSetup.tsx keeps
  PromptEditor, CwdAutocomplete and ModelPicker (new `choices` prop).
- The tool icon moved from the sidebar's workspace row to each session row
  (right edge, `session.tool`); the split-pane header shows the run's tool.
- An ended session opens in its workspace (`&session=`, SessionReplay.tsx):
  its conversation read-only plus a prompt box; only a SENT message resumes
  it (resumeSessionId + prompt, first free slot). The console is only
  hidden behind it.
- Analyst button beside Files/Changes in the right panel opens
  /sessions/:id for the opened session.
- Gemini CLI ships its own icon (client/public/ai-tools/gemini.svg, from
  zonalogo.com).
- RouterPicker always shows (Native disabled for tools without router
  support); useAiRouters() exported.
- Tests: server start-with-tool cases; SessionReplay, home field order/tool/
  router models, empty pane, Analyst; dropped the cwd-field suites
  (Run.defaultCwd, Workspace.laneCwd). Docs: CLAUDE.md rules, API.md,
  LANES.md, README, ARCHITECTURE, INSTALL.

client/src/i18n/locales/en/lanes.json

@@ -317,6 +317,10 @@
   "pane.close": "Stop this pane's session",
   "panel.files": "Files",
   "panel.changes": "Changes",
+  "panel.analyst": "Analyst",
+  "replay.placeholder": "Message to continue this session…",
+  "replay.hint": "Sending resumes the session in a free pane.",
+  "replay.send": "Send",
   "sessions.resume": "Resume",
   "resume.openPane": "Open pane",
   "resume.hint": "Resume this session in a workspace pane — re-reads its whole transcript into context",
@@ -334,7 +338,7 @@
   "tree.allTools": "All tools",
   "tree.newSession": "New session",
   "tree.noSessions": "No sessions yet",
-  "tree.transcriptHint": "No live pane — opens the transcript",
+  "tree.transcriptHint": "No live pane — opens its conversation; a message you send resumes it",
   "tree.splitHint": "Live pane — Ctrl/Cmd+click opens it in a second pane",
   "tree.renameSession": "Rename session",
   "tree.archiveSession": "Archive — hide from this workspace (deletes nothing)",
@@ -346,6 +350,7 @@
   "tree.archived_other": "{{count}} archived sessions",
   "tree.unarchive": "Unarchive",
   "home.breadcrumb": "Workspaces",
+  "home.ai": "AI",
   "home.repoRoot": "Repository root",
   "home.mainBranch": "Main worktree's branch",
   "home.worktreeCount_one": "{{count}} worktree",

client/src/i18n/locales/en/run.json

@@ -1,6 +1,8 @@
 {
+  "emptyPane": "No session in this pane. Start one from the workspace's prompt box.",
   "actions": {
     "newRun": "New run",
+    "newSession": "New session",
     "send": "Send",
     "sending": "Sending…",
     "start": "Run",