test(#1178): consolidate duplicated agent-roster helper into tests/helpers (#1420)

* test(#1178): consolidate duplicated agent-roster helper into tests/helpers

The "list gsd-*.md agent files, strip .md, sort" derivation was hand-duplicated
across the suite (two listAgentFiles(), an identical agentFilesOnDisk(), and
inline readdir blocks). Add tests/helpers/agent-roster.cjs exporting
listAgentFiles(agentsDir?) and route the genuinely-identical source-roster sites
through it. Semantically-different sites (installed-dest dirs, absolute-path
returns, .toml-inclusive Codex rosters, full-.md-filename readers, the uniform
multi-family inventory table) are left intact, each with a one-line comment.

Test-only; no production code touched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test(#1178): note AGENTS_DIR export is for future call sites

Review nit: clarify that the currently-unused AGENTS_DIR export is intentional
— available for future tests needing the canonical source agents path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Tom Boucher <trekkie@nomorestars.com>

tests/helpers/agent-roster.cjs

@@ -0,0 +1,42 @@
+'use strict';
+
+/**
+ * Shared helper for the canonical shipped-agent roster.
+ *
+ * Several tests derive "the set of agents we ship" from the source `agents/`
+ * directory via `fs.readdirSync(...).filter(/^gsd-.*\.md$/)`. This consolidates
+ * that hand-duplicated logic into one canonical SOURCE-roster derivation.
+ *
+ * NOTE: This returns the SOURCE roster (basenames without `.md`, sorted). Sites
+ * with different semantics — installed-destination dirs, absolute-path returns,
+ * or `.toml`-inclusive Codex rosters — must NOT use this helper.
+ */
+
+const fs = require('node:fs');
+const path = require('node:path');
+
+// Canonical source agents directory: <repo-root>/agents, relative to this
+// helper at tests/helpers/. Matches the path the consolidated call sites used.
+const AGENTS_DIR = path.join(__dirname, '..', '..', 'agents');
+
+/**
+ * List shipped agent basenames (without the `.md` extension), sorted.
+ *
+ * @param {string} [agentsDir] Override for the source agents directory.
+ *   Defaults to the canonical `<repo-root>/agents`.
+ * @returns {string[]} Sorted `gsd-*` basenames with `.md` stripped.
+ */
+function listAgentFiles(agentsDir = AGENTS_DIR) {
+  return fs
+    .readdirSync(agentsDir)
+    .filter((f) => /^gsd-.*\.md$/.test(f))
+    .map((f) => f.replace(/\.md$/, ''))
+    .sort();
+}
+
+module.exports = {
+  // AGENTS_DIR is exported (not yet consumed by a call site) so future tests that
+  // need the canonical source agents path can reuse it instead of rediscovering it.
+  AGENTS_DIR,
+  listAgentFiles,
+};