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/components/WorkspaceTree.tsx

@@ -35,10 +35,10 @@ import { DestructiveLaneModal } from "./lanes/DestructiveLaneModal";
 
 /** The workspace (and attached run) the Workspace page shows, from its URL. */
 function selectedFrom(pathname: string, search: string) {
-  if (!pathname.startsWith("/workspace")) return { lane: null, run: null };
+  if (!pathname.startsWith("/workspace")) return { lane: null, run: null, session: null };
   const p = new URLSearchParams(search);
   const v = p.get("lane");
-  return { lane: v === null ? null : Number(v), run: p.get("run") };
+  return { lane: v === null ? null : Number(v), run: p.get("run"), session: p.get("session") };
 }
 
 export function WorkspaceTree() {
@@ -240,7 +240,7 @@ export function WorkspaceTree() {
               <WorkspaceRow
                 lane={lane}
                 index={i}
-                selected={selected.lane === lane.id && !selected.run}
+                selected={selected.lane === lane.id && !selected.run && !selected.session}
                 expanded={expanded(lane.id)}
                 renaming={renamingId === lane.id}
                 onSelect={() => {
@@ -262,6 +262,7 @@ export function WorkspaceTree() {
                 // A workspace found by its name shows all of its sessions.
                 query={q && nameHit(lane) ? "" : query}
                 selectedRunId={selected.lane === lane.id ? selected.run : null}
+                selectedSessionId={selected.lane === lane.id ? selected.session : null}
                 onMatches={q ? (n) => onMatches(lane.id, n) : undefined}
                 hidden={!visible(lane)}
               />