feat(#972): graphify command cutover — first capability owning a command family (ADR-857 phase 4d-impl-2) (#975)

Migrate graphify into a Capability that owns its `graphify` command family,
dispatched via the registry (#961 mechanism) instead of a hardcoded case.
graphify is now an enable/disable plug-in.

- src/graphify-command-router.cts: routeGraphifyCommand (standard route*Command),
  reproduces the removed case EXACTLY (query +--budget, status, diff, build,
  hidden build snapshot, usage/unknown errors); injectable _graphify test seam.
- capabilities/graphify/capability.json: role feature, tier:full, skills:[graphify],
  config:{graphify.enabled default false}, commands:[{family:graphify, module,
  router:routeGraphifyCommand}].
- removed case 'graphify' from gsd-tools.cjs; graphify now flows
  default -> dispatchCapabilityCommand -> commandFamilies.graphify -> router.
- regenerated registry (commandFamilies/bySkill/configSchema/profileMembership/
  capabilityClusters for graphify); tier:full keeps 4c install/surface a no-op.

Equivalence-proven: 8 recording-mock unit tests assert the exact fn+args per
subcommand (budget, snapshot-vs-build); 9 subprocess tests assert distinguishing
output shapes; existing graphify tests pass unchanged through the new path.

Surfaced (not silently accepted): the pre-existing --budget-no-value NaN no-op
quirk, preserved for equivalence, filed separately.

Closes #972

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

capabilities/graphify/capability.json

@@ -0,0 +1,28 @@
+{
+  "id": "graphify",
+  "role": "feature",
+  "title": "Knowledge graph",
+  "description": "Build, query, and inspect the project knowledge graph in `.planning/graphs/`; exposes graphify CLI subcommands (build, query, status, diff) and the /gsd-graphify skill.",
+  "tier": "full",
+  "requires": [],
+  "skills": ["graphify"],
+  "agents": [],
+  "config": {
+    "graphify.enabled": {
+      "type": "boolean",
+      "default": false,
+      "description": "Enable the graphify knowledge-graph command + skill."
+    }
+  },
+  "commands": [
+    {
+      "family": "graphify",
+      "module": "graphify-command-router.cjs",
+      "router": "routeGraphifyCommand"
+    }
+  ],
+  "hooks": [],
+  "steps": [],
+  "contributions": [],
+  "gates": []
+}