test(W7h): retire the literal 75 KB autoplan budget duplicate

W7h retirement (t04 F18). test/skill-preflight-budget.test.ts asserted the
same four-skill autoplan sum twice: once against
AUTOPLAN_PREFLIGHT_BUDGET_BYTES * 3 (25,600 x 3 = 76,800 = 75 KiB) and once
against a literal 75 * 1024, which would drift silently if the constant
changed.

Retirement card:
- test: skill-preflight-budget.test.ts 'autoplan total under 25 KB
  instruction cap × 3 (75 KB instruction budget)'.
- detects: the four plan-* skills' brain preflight + refresh + write-back
  instructions exceeding 75 KiB.
- non_test_callers: none.
- stronger_proof / replacement: 'autoplan: sum across 4 plan-* skills stays
  under AUTOPLAN_PREFLIGHT_BUDGET_BYTES × 3 (instructions)' in the same file,
  which reads the constant from scripts/brain-cache-spec.ts.
- history: c118e24 v1.64.1.0.
- validation: bun test test/skill-preflight-budget.test.ts.

Controls (scratch worktree): shrinking AUTOPLAN_PREFLIGHT_BUDGET_BYTES below
the current total, or growing the eng write-back instructions by 80 KB, turns
the kept constant-based check red (2/2). Log:
.context/mutation-skill-preflight-budget.md.

test/skill-preflight-budget.test.ts

@@ -77,14 +77,6 @@ describe('per-skill preflight token budget', () => {
 });
 
 describe('autoplan total preflight budget (T21 / D7)', () => {
-  test('autoplan total under 25 KB instruction cap × 3 (75 KB instruction budget)', () => {
-    const autoplanSkills = ['plan-ceo-review', 'plan-eng-review', 'plan-design-review', 'plan-devex-review'];
-    const total = autoplanSkills.reduce((sum, s) => sum + totalBrainBytes(s), 0);
-    // The 75 KB cap on instructions across the 4-skill autoplan; runtime
-    // digest budget is the lower 25 KB cap, separately tested above.
-    expect(total).toBeLessThan(75 * 1024);
-  });
-
   test('per-skill subset emits its expected entity references in the preflight block', () => {
     for (const [skill, subset] of Object.entries(SKILL_DIGEST_SUBSETS)) {
       const preflight = generateBrainPreflight(buildCtx(skill));