feat(runner): install stage with progress events

Install stage: package-manager detection (lockfile beats the
packageManager field, npm default — vite-kitchen's shape), install run
inside the clone with staged status heartbeats, and every failure —
including install-before-clone — mapped to the named install-failed
cause with a bounded output tail. The spawned-runner suite now runs the
full clone → install pipeline on the daemon-served vite-kitchen (real
npm registry install) and greps the child's entire stdout+stderr plus
the clone's .git/config for the PAT and workspace token. Shared test
doubles extracted (exec.mock.ts, tests/runner-session.ts) to keep the
suites duplication-free.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

apps/runner/src/server.ts

@@ -11,6 +11,7 @@ import {
 import { runCommand } from "./exec";
 import type { Logger } from "./log";
 import { cloneRepository } from "./stages/clone";
+import { installDependencies } from "./stages/install";
 import { runStage, type StageEmitter } from "./stages/stage-runner";
 
 /**
@@ -142,10 +143,9 @@ export type CommandMessage = Extract<
 
 /** Stages that later walking-skeleton stories wire up (05+). */
 const NOT_IMPLEMENTED: Record<
-  Exclude<CommandMessage["type"], "clone">,
+  Exclude<CommandMessage["type"], "clone" | "install">,
   { cause: RunnerErrorCause; stage?: RunnerStage }
 > = {
-  install: { cause: "install-failed", stage: "installing" },
   scan: { cause: "scan-failed", stage: "scanning" },
   "adapter-start": { cause: "adapter-start-failed", stage: "starting-preview" },
   "render-frame": { cause: "render-failed" },
@@ -184,6 +184,20 @@ async function handleCommand(
       if (result.ok) ctx.ready();
       return;
     }
+    case "install": {
+      const result = await runStage(
+        "installing",
+        "install-failed",
+        stageOptions,
+        installDependencies({
+          workspaceDir: options.workspaceDir,
+          runCommand,
+          log: options.log,
+        }),
+      );
+      if (result.ok) ctx.ready();
+      return;
+    }
     default: {
       const notImplemented = NOT_IMPLEMENTED[command.type];
       emit.error(