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).

docs/reference/capability-matrix.md

@@ -71,7 +71,7 @@ points.
 | `tdd` | feature | full | `>=1.6.0` | `plan:pre`, `execute:post` | contribution, gate | first-party |
 | `ui` | feature | full | `>=1.6.0` | `plan:pre`, `execute:wave:post`, `verify:post` | step, gate | first-party |
 
-### Runtime capabilities (role: runtime) — 15
+### Runtime capabilities (role: runtime) — 16
 
 Runtime capabilities adapt GSD to a specific AI runtime or IDE — emitting
 skills, agents, hooks configuration, and surface files for that host. They
@@ -95,6 +95,7 @@ emission), so their extension-point and hook-kind cells are `—`.
 | `qwen` | runtime | core | `>=1.6.0` | — | — | first-party |
 | `trae` | runtime | core | `>=1.6.0` | — | — | first-party |
 | `windsurf` | runtime | core | `>=1.6.0` | — | — | first-party |
+| `zcode` | runtime | core | `>=1.6.0` | — | — | first-party |
 
 ---