feat(#1304): add optional activationKey capability manifest field (#1309)

Add an optional activationKey to the feature role of capability.json — the
dotted config key that gates the whole capability (e.g. graphify.enabled).
gen-capability-registry validates it (non-empty string, reserved-name guard,
must be declared in the capability's own config slice, feature-only) and emits
it per-capability in the generated registry. Declared on graphify + intel.
No runtime consumption yet (resolver wiring lands in #1305). Part of #1302.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

capabilities/graphify/capability.json

@@ -8,6 +8,7 @@
   "runtimeCompat": { "supported": ["*"], "unsupported": [] },
   "skills": ["graphify"],
   "agents": [],
+  "activationKey": "graphify.enabled",
   "config": {
     "graphify.enabled": {
       "type": "boolean",