review: clarify diff-coverage nudge gives explicit license to skip generated artifacts

the one-time pre-flight nudge said "optionally read" but never told the agent
it's free to retry without reading when every unread region is generated
(lockfiles, codegen, snapshots, migration metadata). audit #677 surfaced ~21
runs/24h burning an extra model turn re-reading drizzle snapshots, pnpm-lock,
and *.gen.ts files purely to satisfy the gate. mode prompts only mention
generated content in the "skip self-review entirely" path, not the
"in-progress substantive review" path, so the in-the-moment error message
was the gap. behavior unchanged for legitimately-unread source regions.

mcp/review.ts

@@ -712,7 +712,9 @@ function runDiffCoveragePreflight(params: { ctx: ToolContext }): void {
     .join("\n");
   throw new Error(
     `diff coverage pre-flight: some TOC regions were not read before review submission. ` +
-      `this is a one-time nudge — optionally read the ranges below from ${coverageState.diffPath}, then call create_pull_request_review again with the same arguments. ` +
+      `this is a one-time nudge — read the ranges below from ${coverageState.diffPath} on a best-effort basis, then call create_pull_request_review again with the same arguments. ` +
+      `you are NOT obligated to read generated artifacts (lockfiles like pnpm-lock.yaml / package-lock.json / yarn.lock / Cargo.lock; codegen output like *.gen.*, *.pb.go, *.generated.*; snapshot/fixture dirs like __snapshots__/; migration metadata like drizzle/meta/, prisma migration SQL). ` +
+      `if every unread region is generated, retry immediately without reading. ` +
       `this pre-flight will not block again in this review session.\n\n` +
       `unread TOC regions:\n${unreadText}\n\n` +
       `${coverageState.lastBreakdown}`