feat(ack): bind test env to deployer and add regression mode

ACK 0.19.0 hands test-environment deploys to the deployer skill,
documents bug-fix as a first-class scenario, and adds
docs/ack/regression.yaml harvest plus a /ack regression run.

skills/ack/templates/delivery.schema.json

@@ -70,8 +70,18 @@
       "additionalProperties": false,
       "properties": {
         "testEnvironment": {
-          "type": ["string", "null"],
-          "pattern": "^[a-z][a-z0-9-]{0,63}$"
+          "oneOf": [
+            { "type": "null" },
+            {
+              "type": "object",
+              "additionalProperties": false,
+              "required": ["via", "env"],
+              "properties": {
+                "via": { "const": "deployer" },
+                "env": { "$ref": "#/definitions/id" }
+              }
+            }
+          ]
         },
         "release": {
           "type": ["string", "null"],