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).

src/runtime-name-policy.cts

@@ -244,6 +244,7 @@ const GLOBAL_CONFIG_HOME_FRAGMENTS: Readonly<Record<string, string>> = {
   codebuddy: "'.codebuddy'",
   cline:     "'.cline'",
   kimi:      "'.config', 'agents'",
+  zcode:     "'.zcode'",
 };
 
 /**