feat(hooks): additionalContext injection advisory + filter-before lever (v5.10 B5) [skip-docs]

HKV now flags hooks that build hookSpecificOutput.additionalContext from
un-grepped command output as an INFO advisory (weight 0, never severity-bearing
— excluded from the self-audit nonInfo set). That field enters Claude's context
every time the hook fires (plain stdout on exit 0 does not), so an unfiltered
payload is a recurring per-turn token cost.

- New lib scanners/lib/hook-additional-context.mjs: pure assessHookAdditionalContext
  (unit-tested, no IO) + IO wrapper assessHookContextForRepo (walk hooks->scripts).
  Heuristic: additionalContext + verbose-prone capture (cat/git log/execSync/…) &&
  no filter (grep/head/jq/.slice). Deliberately low precision -> advisory only.
- HKV: emits the advisory inline on scripts it already reads (after the M5 verbose
  check). Additive, info severity -> frozen v5.0.0 + SC-5 snapshots untouched.
- feature-gap: new filterHookLeverFinding companion, fires ONLY when >=1 chatty
  hook is detected — surfaces the filter-before-Claude-reads lever (CC
  filter-test-output.sh). Silent otherwise (opportunity, not noise).
- docs: README HKV + GAP scanner rows; scanner-internals.md HKV row + full B5
  implementation note. CLAUDE.md kept lean ([skip-docs]); B5 fully documented in
  README + scanner-internals.

Mechanism verified 2026-06-23 against code.claude.com/docs context-window.md
(additionalContext enters context; plain stdout does not). Suite 1239 -> 1254 green.
Version/badges/CHANGELOG wait for the v5.10 release cut.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/scanners/feature-gap-scanner.test.mjs

@@ -5,7 +5,7 @@ import { fileURLToPath } from 'node:url';
 import { mkdtemp, mkdir, writeFile, rm } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
 import { resetCounter } from '../../scanners/lib/output.mjs';
-import { scan, opportunitySummary, bundledSkillsLeverFinding, cliOverMcpLeverFinding } from '../../scanners/feature-gap-scanner.mjs';
+import { scan, opportunitySummary, bundledSkillsLeverFinding, cliOverMcpLeverFinding, filterHookLeverFinding } from '../../scanners/feature-gap-scanner.mjs';
 import { discoverConfigFiles } from '../../scanners/lib/file-discovery.mjs';
 import { withHermeticHome } from '../helpers/hermetic-home.mjs';
 
@@ -364,3 +364,50 @@ describe('cliOverMcpLeverFinding (CLI-over-MCP lever, v5.10 B4)', () => {
     assert.match(f.recommendation || '', /\bgh\b|\baws\b|\bgcloud\b/);
   });
 });
+
+describe('filterHookLeverFinding (filter-before-Claude-reads lever, v5.10 B5)', () => {
+  it('returns null when no chatty hook was detected', () => {
+    assert.equal(filterHookLeverFinding({ flaggedHooks: [] }), null);
+    assert.equal(filterHookLeverFinding({}), null);
+    assert.equal(filterHookLeverFinding(), null);
+  });
+
+  it('fires an info opportunity when ≥1 chatty hook is detected', () => {
+    resetCounter();
+    const f = filterHookLeverFinding({
+      flaggedHooks: [{ event: 'SessionStart', scriptPath: '/x/hooks/scripts/chatty.sh' }],
+    });
+    assert.ok(f, 'expected a lever finding');
+    assert.equal(f.severity, 'info');
+    assert.equal(f.category, 'token-efficiency');
+    assert.match(f.description || '', /filter-test-output\.sh|filter/i);
+    assert.match(f.evidence || '', /chatty_hooks=1/);
+  });
+});
+
+describe('GAP scanner — filter-before lever wiring (chatty hook fixture)', () => {
+  it('emits the filter-before lever when scanning a repo with a chatty hook', async () => {
+    resetCounter();
+    const discovery = await fixtureDiscovery('hooks-additional-context');
+    const result = await withHermeticHome(() =>
+      scan(resolve(FIXTURES, 'hooks-additional-context'), discovery),
+    );
+    const lever = result.findings.find(
+      f => f.scanner === 'GAP' && /chatty_hooks=/.test(f.evidence || ''),
+    );
+    assert.ok(lever, `expected filter-before lever; got: ${result.findings.map(x => x.title).join(' | ')}`);
+    assert.equal(lever.severity, 'info');
+  });
+
+  it('does NOT emit the lever for a repo with only quiet hooks', async () => {
+    resetCounter();
+    const discovery = await fixtureDiscovery('hooks-quiet');
+    const result = await withHermeticHome(() =>
+      scan(resolve(FIXTURES, 'hooks-quiet'), discovery),
+    );
+    const lever = result.findings.find(
+      f => f.scanner === 'GAP' && /chatty_hooks=/.test(f.evidence || ''),
+    );
+    assert.equal(lever, undefined, `expected no filter-before lever; got id=${lever?.id}`);
+  });
+});