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.

examples/workflows/role_pipeline.toml

@@ -1,4 +1,4 @@
-# Role pipeline: analyst → architect → planner → implementer ⇄ reviewer
+# Role pipeline: analyst → architect → brief_echo → planner → implementer ⇄ reviewer
 #
 # Each stage produces a typed artifact that the next stage `needs`, so work flows forward
 # without a human relaying notes. The decision journal (pinned into every stage's context)
@@ -18,6 +18,8 @@
 #   id = "impl_plan";     schema_path = "schemas/impl_plan.json";     llm_emitted = true
 #   [[artifacts]]
 #   id = "review_report"; schema_path = "schemas/review_report.json"; llm_emitted = true
+#   [[artifacts]]
+#   id = "brief_echo";    schema_path = "schemas/brief_echo.json";    llm_emitted = true
 #
 # Prompt files (prompts/*.md, relative to this workflow) must exist for a real run.
 
@@ -45,7 +47,20 @@ produces = [{ name = "design", kind = "design" }]
 token_budget = 16384
 max_retries = 2
 
-# 3. Break the design into ordered, verifiable steps.
+# 3a. Before planning: echo the analyst brief back as a structured artifact.
+#     The brief_echo gate diffs the restatement against the original analysis;
+#     on divergence (dropped requirements or hallucinated files) it fails the
+#     stage (retryable) so the pipeline cannot reach plan generation on a misread brief.
+[[stages]]
+id = "brief_echo"
+prompt = "prompts/brief_echo.md"
+needs = ["analysis"]
+produces = [{ name = "brief_echo", kind = "brief_echo" }]
+brief_echo = true
+token_budget = 8192
+max_retries = 2
+
+# 3b. Break the design into ordered, verifiable steps.
 [[stages]]
 id = "planner"
 prompt = "prompts/planner.md"
@@ -90,12 +105,19 @@ condition_type = "artifact_validated"
 condition_artifact_id = "analysis"
 
 [[transitions]]
-id = "architect-to-planner"
+id = "architect-to-brief-echo"
 from = "architect"
-to = "planner"
+to = "brief_echo"
 condition_type = "artifact_validated"
 condition_artifact_id = "design"
 
+[[transitions]]
+id = "brief-echo-to-planner"
+from = "brief_echo"
+to = "planner"
+condition_type = "artifact_validated"
+condition_artifact_id = "brief_echo"
+
 [[transitions]]
 id = "planner-to-implementer"
 from = "planner"