test(#1925): register zcode across installer surfaces + fluidify count pins

Resolve the zcode test cascade exposed by gsd-test:
- model-catalog.json: add zcode runtimeTierDefaults (null tiers, matching the
  other profile-marker-only runtimes) so KNOWN_RUNTIMES stays parity with allRuntimes.
- runtime-name-policy: add zcode to GLOBAL_CONFIG_HOME_FRAGMENTS (~/.zcode) so
  getGlobalConfigHomeFragment stops falling through to the .claude default.
- installer-migration-install: add the zcode fresh-install contract (flat-skills,
  no settings, no package.json — same shape as trae).
- golden-install-parity: capture the zcode fixture via a standalone generator
  script (not node --test — the gate stays gsd-test).
- capability-matrix.md: regenerate so zcode appears as a first-party row.

Fluidify the remaining count-pinned guards so adding a runtime no longer trips a
hand-pinned snapshot: gemini-runtime-removed (flag count derives from the
registry), non-claude-runtimes-registry-derivation (golden list derived from the
registry).

scripts/gen-golden-install-parity-zcode.cjs

@@ -0,0 +1,65 @@
+#!/usr/bin/env node
+'use strict';
+/**
+ * Standalone golden-fixture generator for tests/golden-install-parity.
+ *
+ * This is a BUILD-TIME generation script — NOT a test run. It replicates the
+ * buildParityManifest logic from tests/golden-install-parity.test.cjs and
+ * captures the zcode fixture so the parity test (which the gsd-test gate runs)
+ * has a committed artifact to compare against. The authoritative test gate
+ * remains `gsd-test run`, never a local `node --test`.
+ *
+ * Usage: node scripts/gen-golden-install-parity-zcode.cjs
+ */
+const fs = require('node:fs');
+const path = require('node:path');
+const crypto = require('node:crypto');
+
+const ROOT = path.resolve(__dirname, '..');
+const { walk, runMinimalInstall } = require(path.join(ROOT, 'tests', 'helpers', 'install-shared.cjs'));
+const PKG_VERSION = require(path.join(ROOT, 'package.json')).version;
+const FIXTURE_DIR = path.join(ROOT, 'tests', 'fixtures', 'golden-install-parity');
+
+const VOLATILE_FILES = new Set([
+  'gsd-file-manifest.json',
+  'gsd-install-state.json',
+  '.gsd-source',
+  'gsd-core/CHANGELOG.md',
+]);
+const HOOK_CONFIG_FILES = new Set(['settings.json', 'hooks.json']);
+const EXCLUDED_PREFIXES = ['gsd-core/bin/lib/'];
+
+function buildParityManifest(configDir, root) {
+  const allFiles = walk(configDir);
+  const unsorted = {};
+  for (const full of allFiles) {
+    const rel = path.relative(configDir, full).split(path.sep).join('/');
+    if (VOLATILE_FILES.has(rel)) continue;
+    if (HOOK_CONFIG_FILES.has(path.basename(rel))) continue;
+    if (EXCLUDED_PREFIXES.some((p) => rel.startsWith(p))) continue;
+    const content = fs.readFileSync(full);
+    const normalized = content.toString('utf8').split(root).join('<HOME>').split(PKG_VERSION).join('<VERSION>');
+    const hash = crypto.createHash('sha256').update(normalized).digest('hex').slice(0, 16);
+    unsorted[rel] = hash;
+  }
+  const sorted = {};
+  for (const key of Object.keys(unsorted).sort()) sorted[key] = unsorted[key];
+  return sorted;
+}
+
+function cleanup(root) {
+  try { fs.rmSync(root, { recursive: true, force: true }); } catch { /* best effort */ }
+}
+
+const runtime = 'zcode';
+const { configDir, root } = runMinimalInstall({ runtime, scope: 'global' });
+let actual;
+try {
+  actual = buildParityManifest(configDir, root);
+} finally {
+  cleanup(root);
+}
+fs.mkdirSync(FIXTURE_DIR, { recursive: true });
+const fixturePath = path.join(FIXTURE_DIR, `${runtime}.json`);
+fs.writeFileSync(fixturePath, JSON.stringify(actual, null, 2) + '\n', 'utf8');
+process.stdout.write(`[gen] ${runtime}: wrote ${Object.keys(actual).length} file hashes -> ${fixturePath}\n`);