P1 spine: monorepo, shared WS protocol, server daemon

- npm workspaces (shared / server / web), TS strict, project refs
- @arboretum/shared: multiplexed WS protocol (JSON control + binary
  output frames: 1B type + u32le channel), flow-control constants
  (ACK 256K, HIGH 384K, LOW 128K, lagging 2M), REST types
- git-arboretum server: Fastify 5 + node:sqlite (single native dep:
  node-pty prebuilt), token auth (sha256 at rest, HMAC cookie, global
  login rate limit + backoff), strict Origin check on /api and /ws,
  PtyManager (2MiB ring with monotonic offset, resync replay = reset +
  256KiB tail, pause/resume only when ALL interactive clients exceed
  HIGH, observers never throttle, lagging clients resync), WS gateway
  (attach/stdin/resize/ack, heartbeat 30s), SIGTERM→SIGKILL 5s grace
- CLI: arboretum [--port 7317] [--bind 127.0.0.1] — non-loopback bind
  requires an explicit safety flag
- Smoke-tested: login/401/403-origin/spawn bash/kill/grace-SIGKILL all
  green

packages/server/src/core/claude-launcher.ts

@@ -0,0 +1,34 @@
+import { execFileSync } from 'node:child_process';
+
+export interface SpawnSpec {
+  file: string;
+  args: string[];
+  env: NodeJS.ProcessEnv;
+}
+
+let cachedClaudeBin: string | null = null;
+
+export function resolveClaudeBin(): string {
+  if (cachedClaudeBin) return cachedClaudeBin;
+  try {
+    cachedClaudeBin = execFileSync('which', ['claude'], { encoding: 'utf8' }).trim();
+  } catch {
+    throw new Error(
+      'Claude Code CLI not found in PATH. Install it first: https://code.claude.com/docs/en/quickstart',
+    );
+  }
+  return cachedClaudeBin;
+}
+
+/** Module volontairement abstrait : le plan B « BYO API key / Agent SDK » se brancherait ici. */
+export function buildSpawnSpec(command: 'claude' | 'bash'): SpawnSpec {
+  const env: NodeJS.ProcessEnv = {
+    ...process.env,
+    TERM: 'xterm-256color',
+    COLORTERM: 'truecolor',
+  };
+  if (command === 'bash') {
+    return { file: 'bash', args: ['--norc'], env };
+  }
+  return { file: resolveClaudeBin(), args: [], env };
+}