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).

tests/installer-migration-install.integration.test.cjs

@@ -40,6 +40,7 @@ const RUNTIME_INSTALL_CONTRACTS = {
   qwen: { surface: 'flat-skills', settings: true, packageJson: true },
   trae: { surface: 'flat-skills', settings: false, packageJson: false },
   windsurf: { surface: 'global-artifacts-noop', settings: false, packageJson: false },
+  zcode: { surface: 'flat-skills', settings: false, packageJson: false },
 };
 
 function sha256(content) {