feat(kernel): brief echo-back gate (plan-pipeline-addenda A1)

New brief_echo stage before the planner in role_pipeline: the model restates
the analyst brief as a structured artifact, and a deterministic gate
(checkBriefEcho, opt-in via brief_echo=true) diffs the restatement against the
original brief. On divergence — dropped requirements (Jaccard coverage < 0.34)
or hallucinated files — it emits BriefEchoMismatchEvent and fails the stage
retryably, so a misread brief never reaches plan generation.

The diff (BriefEchoDiff) is a pure function of two recorded artifacts, so it is
replay-safe and records no observation; the event fires only on mismatch, for
audit. Symbols are recorded but non-blocking in v1. Mirrors the groundBrief-
References gate. role_pipeline only; freestyle_planning wiring is a follow-up.

Runtime install (like research): copy brief_echo.json + brief_echo.md and the
[[artifacts]] block into ~/.config/correx.

docs/schemas/brief_echo.json

@@ -0,0 +1,27 @@
+{
+  "type": "object",
+  "properties": {
+    "goal": {
+      "type": "string",
+      "description": "the analyst brief's goal restated in your own words"
+    },
+    "referenced_files": {
+      "type": "array",
+      "description": "workspace-relative file paths named in the brief, one per item",
+      "items": { "type": "string" }
+    },
+    "referenced_symbols": {
+      "type": "array",
+      "description": "class, interface, or function names named in the brief, one per item",
+      "items": { "type": "string" }
+    },
+    "acceptance_criteria": {
+      "type": "array",
+      "description": "the brief's requirements / acceptance criteria restated faithfully, one per item",
+      "minItems": 1,
+      "items": { "type": "string" }
+    }
+  },
+  "required": ["goal", "acceptance_criteria"],
+  "additionalProperties": true
+}