feat(integrations): add Grok Build skills-based integration (#3535)

* feat(integrations): add Grok Build skills-based integration

Add first-class support for xAI Grok Build via SkillsIntegration, installing
speckit skills under .grok/skills and wiring init/invocation/catalog surfaces.

Assisted-by: Grok Build (model: grok-4, supervised)

* test+docs: address Copilot review on Grok multi-install and next steps

Assert init next-steps guidance for Grok (.grok/skills, /speckit-*) and
clarify that multi-install safety is path/manifest isolation, not
agent-context defaults such as shared AGENTS.md.

* fix(integrations): Grok headless --always-approve and isolation paths

Document Grok multi-install isolation as .grok/skills and .grok/rules.
Override build_exec_args to pass --always-approve so non-interactive
dispatch is not blocked at tool permission gates.

* docs(integrations): list only managed .grok/skills for Grok isolation

Multi-install isolation documents Spec Kit-managed paths; Grok only
writes .grok/skills, so drop the read-only .grok/rules entry.

* fix(integrations): always-slash Grok hooks and refresh catalog date

Move grok to ALWAYS_SLASH_AGENTS so hooks never emit /speckit.plan when
ai_skills is missing/false. Update slash-format tests, persist ai_skills
on init, and bump catalog updated_at for the Grok entry.

---------

Co-authored-by: Nate Chadwick <1232206+natechadwick@users.noreply.github.com>
Co-authored-by: test <test@example.com>

integrations/catalog.json

@@ -1,6 +1,6 @@
 {
   "schema_version": "1.0",
-  "updated_at": "2026-06-23T00:00:00Z",
+  "updated_at": "2026-07-15T00:00:00Z",
   "catalog_url": "https://raw.githubusercontent.com/github/spec-kit/main/integrations/catalog.json",
   "integrations": {
     "claude": {
@@ -282,6 +282,15 @@
       "repository": "https://github.com/github/spec-kit",
       "tags": ["cli"]
     },
+    "grok": {
+      "id": "grok",
+      "name": "Grok Build",
+      "version": "1.0.0",
+      "description": "xAI Grok Build CLI skills-based integration",
+      "author": "spec-kit-core",
+      "repository": "https://github.com/github/spec-kit",
+      "tags": ["cli", "skills", "xai"]
+    },
     "hermes": {
       "id": "hermes",
       "name": "Hermes Agent",