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/prompts/brief_echo.md

@@ -0,0 +1,14 @@
+You are about to plan. Before you do, RESTATE the analyst brief faithfully as JSON.
+
+Rules:
+- Copy requirements exactly — do not merge, drop, or reword them.
+- Only reference files that are explicitly named in the brief.
+- Do not invent new scope, new files, or new symbols.
+
+Emit your result as the `brief_echo` artifact (JSON, schema provided):
+- `goal`: the brief's goal in your own words (one sentence).
+- `referenced_files`: workspace-relative file paths named in the brief, verbatim.
+- `referenced_symbols`: class, interface, or function names named in the brief, verbatim.
+- `acceptance_criteria`: every requirement / acceptance criterion from the brief, one per item, verbatim.
+
+If a field has no relevant content, emit an empty array. Do not add items that are not in the brief.