fix(setup): --host slate exits informatively instead of silently installing nothing

slate passed --host validation (added to the accept-list in v1.64.1.0) but never got a dispatch arm, and the all-INSTALL_*-zero fallback lives inside the auto branch — so './setup --host slate' configured nothing and exited 0, a silent no-op strictly worse than the original hard rejection. slate is now an informational arm (per docs/designs/SLATE_HOST.md it is blocked on the host-config refactor; Slate reads .claude/skills as a compatibility fallback, so the arm points at './setup --host claude'), and a defensive guard after the dispatch chain errors loudly (naming the host, the missing arm, and the valid targets, exit 1) if a future host is ever accepted without being wired.

Regression tests (fail on v1.68.3.0): a dispatch-arm ratchet asserting every accept-listed install target has a matching dispatch branch — the exact drift class; a registry cross-check deriving both sides from hosts/index.ts and setup's case arms; a behavioral slate probe (exit 0, points at --host claude, never reaches the installer — on unfixed code it fell through into the installer); and a static pin on the guard's shape.

Fixes #2361

test/setup-help.test.ts

@@ -62,3 +62,67 @@ describe('setup: --help flag (#1133)', () => {
     expect(res.stdout).toContain('Usage:');
   });
 });
+
+describe('setup: host accept-list ↔ hosts/index.ts registry cross-check (#2361)', () => {
+  // The #2361 failure class: a host passes --host validation but has no
+  // install arm, so `./setup --host <it>` configures nothing and exits 0.
+  // This cross-check derives BOTH sides — the registry from hosts/index.ts
+  // and the case arms from setup — so adding a host to either place without
+  // the other goes red at the moment of the drift, not in a user report.
+
+  const content = fs.readFileSync(SETUP_SCRIPT, 'utf-8');
+
+  function hostCaseArms(): { installTargets: string[]; namedArms: string[] } {
+    const start = content.indexOf('case "$HOST" in');
+    expect(start).toBeGreaterThan(-1);
+    const block = content.slice(start, content.indexOf('\nesac', start));
+    // The pipe list is the install accept-list; single-name arms are informational.
+    const installTargets: string[] = [];
+    const namedArms: string[] = [];
+    for (const m of block.matchAll(/^ {2}([a-z|]+)\)/gm)) {
+      const names = m[1].split('|');
+      if (names.length > 1) installTargets.push(...names.filter((n) => n !== 'auto'));
+      else if (names[0] !== 'auto') namedArms.push(names[0]);
+    }
+    return { installTargets, namedArms };
+  }
+
+  test('registry names == accept-list (minus auto) + informational arms', async () => {
+    const { ALL_HOST_CONFIGS } = await import('../hosts/index');
+    const registered = ALL_HOST_CONFIGS.map((c: { name: string }) => c.name).sort();
+    const { installTargets, namedArms } = hostCaseArms();
+    const covered = [...new Set([...installTargets, ...namedArms])].sort();
+    expect(covered).toEqual(registered);
+  });
+
+  test('every accept-listed install target has a dispatch arm (the exact #2361 hole)', () => {
+    // Set-membership alone would have passed while slate sat accepted-but-
+    // unwired: the invariant that bites is accept-list ⊆ dispatch arms.
+    const { installTargets } = hostCaseArms();
+    expect(installTargets.length).toBeGreaterThan(0);
+    for (const host of installTargets) {
+      expect(content).toMatch(new RegExp(`\\[ "\\$HOST" = "${host}" \\]`));
+    }
+  });
+
+  test('slate informational arm: explains itself, points at --host claude, exit 0', () => {
+    const res = spawnSync('bash', [SETUP_SCRIPT, '--host', 'slate'], {
+      encoding: 'utf-8',
+      timeout: 5000,
+    });
+    expect(res.status).toBe(0);
+    expect(res.stdout).toContain('./setup --host claude');
+    expect(res.stdout).toContain('.claude/skills');
+    // It must not fall through into the installer.
+    expect(res.stdout).not.toMatch(/Installing|bun install|Building/);
+  });
+
+  test('zero-dispatch guard exists: unwired host errors loudly instead of exit-0 no-op', () => {
+    // The guard is only reachable when a future host is accepted but unwired,
+    // so pin its presence and shape statically: it must name the host, call
+    // itself a setup bug, and exit 1.
+    const guard = content.match(/no install arm exists for host[^\n]*\n\s*exit 1/);
+    expect(guard).toBeTruthy();
+    expect(content).toContain("[ \"$HOST\" != \"auto\" ]");
+  });
+});