fix(jobs): backlinks worker defaults to check, not fix (#1853)

Backlinks Minion jobs submitted with an empty payload (the
sync→embed→backlinks chains enqueued after every ingestion) defaulted to
action='fix', rewriting tracked brain pages with generated "Referenced in"
timeline bullets on every routine run — 129 vault files polluted in one
day on our production brain before we traced it.

This contradicts the documented intent in src/core/cycle.ts
(runPhaseBacklinks): "Maintenance cycles must not rewrite tracked brain
pages with generated 'Referenced in' timeline bullets. [...] the legacy
filesystem fixer remains available explicitly via `gbrain check-backlinks
fix`." — the jobs-worker handler simply inverted that default.

Fix: default to 'check'; 'fix' requires explicit opt-in via
'{"action":"fix"}' (the documented submit shape) or
`gbrain check-backlinks fix`. Both explicit paths are unchanged.

Adds a structural regression test (fix-wave-structural.test.ts precedent)
pinning the default, since the handler dynamically imports
runBacklinksCore and walks a real repo dir — a behavioral test would
require mocking that hides the regression behind a test seam.

Co-authored-by: Valentin Ferriere <valentin@v-labs.fr>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/commands/jobs.ts

@@ -1664,7 +1664,13 @@ export async function registerBuiltinHandlers(
 
   worker.register('backlinks', async (job) => {
     const { runBacklinksCore } = await import('./backlinks.ts');
-    const action: 'check' | 'fix' = job.data.action === 'check' ? 'check' : 'fix';
+    // Default to 'check', not 'fix': backlinks jobs submitted with an empty
+    // payload (e.g. the sync→embed→backlinks chains enqueued after ingestion)
+    // must never rewrite tracked brain pages with generated "Referenced in"
+    // timeline bullets. Mirrors the documented intent in src/core/cycle.ts
+    // (runPhaseBacklinks). The filesystem fixer stays available explicitly
+    // via '{"action":"fix"}' or `gbrain check-backlinks fix`.
+    const action: 'check' | 'fix' = job.data.action === 'fix' ? 'fix' : 'check';
     const dir = typeof job.data.dir === 'string'
       ? job.data.dir
       : (await engine.getConfig('sync.repo_path')) ?? '.';

test/backlinks-job-default.test.ts

@@ -0,0 +1,47 @@
+/**
+ * Structural regression for the backlinks Minion handler default.
+ *
+ * Backlinks jobs submitted with an EMPTY payload (the sync→embed→backlinks
+ * chains enqueued after every ingestion) must run as 'check', never 'fix'.
+ * The pre-fix handler inverted the default (`=== 'check' ? 'check' : 'fix'`),
+ * so every routine post-ingestion job rewrote tracked brain pages with
+ * generated "Referenced in" timeline bullets — contradicting the documented
+ * intent in src/core/cycle.ts (runPhaseBacklinks): "Maintenance cycles must
+ * not rewrite tracked brain pages with generated 'Referenced in' timeline
+ * bullets."
+ *
+ * Source-grep is the right tool here (see fix-wave-structural.test.ts): the
+ * handler dynamically imports runBacklinksCore and walks a real repo dir, so
+ * a behavioral test would require heavy mocking that hides the regression
+ * behind a test seam. The rule is "this specific default must stay 'check'".
+ */
+import { describe, test, expect } from 'bun:test';
+import { readFileSync } from 'fs';
+
+describe('backlinks Minion handler — empty payload defaults to check, not fix', () => {
+  const src = readFileSync('src/commands/jobs.ts', 'utf8');
+
+  // Isolate the backlinks register block so assertions can't accidentally
+  // match another handler's action parsing.
+  const blockMatch = src.match(
+    /worker\.register\('backlinks',[\s\S]*?runBacklinksCore\(\{[\s\S]*?\}\);/
+  );
+
+  test('the backlinks handler block exists', () => {
+    expect(blockMatch).not.toBeNull();
+  });
+
+  test("default action is 'check' (explicit opt-in required for 'fix')", () => {
+    const block = blockMatch![0];
+    expect(block).toMatch(
+      /job\.data\.action\s*===\s*'fix'\s*\?\s*'fix'\s*:\s*'check'/
+    );
+  });
+
+  test('the inverted (fix-by-default) shape stays absent', () => {
+    const block = blockMatch![0];
+    expect(block).not.toMatch(
+      /job\.data\.action\s*===\s*'check'\s*\?\s*'check'\s*:\s*'fix'/
+    );
+  });
+});