Install /captain only on Capy machines (GST-1833) (#4143)

* Load the Capy crew skills only on Capy machines (GST-1833)

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* Add changelog entry for Capy-only crew skills (GST-1833)

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* Test that the harness registrar leaves /captain out off Capy

Review-Finding: p1/specialist-batched#2 reader=testing
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* Say in the skills guide that /captain installs only on Capy machines

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

---------

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

scripts/discover-skills.ts

@@ -4,6 +4,7 @@
  */
 
 import * as fs from 'fs';
+import * as os from 'os';
 import * as path from 'path';
 import type { HostConfig } from './host-config';
 
@@ -20,6 +21,23 @@ const SKIP = new Set(['node_modules', '.git', 'dist']);
  */
 export const RETIRED_SKILLS = ['plan-eng-review', 'investigate'];
 
+/**
+ * The gstack-on-Capy skills (crew/README.md): registered only on a Capy
+ * machine, and kept off every host elsewhere like a retired skill. Same list
+ * as GSTACK_CREW_SKILLS in bin/gstack-install-registry.sh, and the same Capy
+ * test as its gstack_on_capy.
+ */
+export const CREW_SKILLS = ['captain'];
+
+export function onCapyMachine(env: NodeJS.ProcessEnv = process.env, home = os.homedir()): boolean {
+  return Boolean(env.CAPY_MACHINE_CONFIG) || fs.existsSync(path.join(home, '.capy', 'system'));
+}
+
+/** What no host registers on this machine: the retired skills, plus the crew skills off Capy. */
+export function unregisteredSkills(env: NodeJS.ProcessEnv = process.env, home = os.homedir()): string[] {
+  return onCapyMachine(env, home) ? RETIRED_SKILLS : [...RETIRED_SKILLS, ...CREW_SKILLS];
+}
+
 /** The generator and its coverage checks use the same include-minus-skip rule. */
 export function includesSkill(host: HostConfig, skillDir: string): boolean {
   const { includeSkills, skipSkills } = host.generation;