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/install-minimal-hooks.test.cjs

@@ -393,6 +393,8 @@ describe('install: on-disk skill files match manifest for --minimal', () => {
           const onDisk = collectSkillBasenamesOnDisk(configDir);
           const inManifest = manifestSkillSet(manifest);
           assert.deepStrictEqual([...onDisk].sort(), [...inManifest].sort());
+          // Not the shared listAgentFiles() helper: asserts on the INSTALLED
+          // dest dir (must be empty in --minimal mode), not the source roster.
           const agentsDir = path.join(configDir, 'agents');
           if (fs.existsSync(agentsDir)) {
             const gsdAgents = fs.readdirSync(agentsDir)