Fix two Bugbot findings: Cursor prefix budget and footer-cutting tail slice

Both flagged by Cursor Bugbot on PR #508 after the main merge; both real.

1. cursorBlockMessage computed min(maxChars, 4000 - prefix), so a
   configured maxChars at or below the Cursor ceiling never charged the
   BLOCK_PREFIX against the budget: the final deny text could exceed
   maxChars by the prefix length, and appendDesignSystemNoteOnce's size
   check lost exactly the room designNoteReserve had held back. The
   prefix now comes off whichever limit binds. Default-config behavior
   is unchanged (min(8000, 4000) - 60 equals the old 4000 - 60).

2. The note reservation is subtracted after renderTemplate's 500-char
   floor, so the clamp can run below the budget clampLastLine assumed
   safe, and its last-resort path tail-sliced the rendered text, cutting
   the policy footer (the failure mode this PR exists to eliminate) when
   a deep file path met a pending DESIGN.md note. The reservation order
   stays (the staleness-note delivery guarantee at floor budgets depends
   on it); the last resort now drops the finding line and clips the head
   instead, so the footer survives every path. New regression test pins
   it: 6 findings, 100-char path, maxChars 500, reserveChars 134.

Prepared with AI assistance (Claude Code).

Co-Authored-By: Claude <noreply@anthropic.com>

skill/scripts/hook-before-edit.mjs

@@ -358,10 +358,16 @@ const BLOCK_PREFIX = 'Impeccable design hook blocked this write before it landed
 
 function cursorBlockMessage(findings, filePath, config, cwd, footerMode, reserveChars) {
   const limits = config?.limits || DEFAULT_CONFIG.limits;
+  // Subtract the prefix from whichever limit binds, not just the Cursor
+  // ceiling: with a configured maxChars at or below the ceiling, the old
+  // `min(maxChars, ceiling - prefix)` never charged the prefix against the
+  // budget, so the final deny text could exceed maxChars by the prefix
+  // length and appendDesignSystemNoteOnce's size check lost exactly the
+  // room designNoteReserve had held back (Bugbot on PR #508).
   const budget = Math.min(
     limits.maxChars || DEFAULT_CONFIG.limits.maxChars,
-    CURSOR_DENY_LIMIT - BLOCK_PREFIX.length,
-  );
+    CURSOR_DENY_LIMIT,
+  ) - BLOCK_PREFIX.length;
   const rendered = renderTemplate(findings, filePath,
     { ...config, limits: { ...limits, maxChars: budget } },
     { cwd, footer: footerMode, reserveChars });

skill/scripts/hook-lib.mjs

@@ -975,7 +975,10 @@ export function renderTemplate(findings, filePath, config, opts = {}) {
   const cap = Math.max(1, limits.maxFindings || DEFAULT_CONFIG.limits.maxFindings);
   // reserveChars holds back room for a note the caller appends after render
   // (the DESIGN.md staleness note), so the final payload stays inside the
-  // configured budget.
+  // configured budget. It comes off after the 500-char floor, so at floor
+  // configs the note keeps guaranteed delivery room; the clamp budget can
+  // therefore sit below 500, which clampLastLine's footer-preserving
+  // fallback handles (Bugbot on PR #508).
   const maxChars = Math.max(500, limits.maxChars || DEFAULT_CONFIG.limits.maxChars) - (opts.reserveChars || 0);
 
   const cwd = opts.cwd || process.cwd();
@@ -1115,15 +1118,22 @@ function clampToBudget(header, lines, more, footer, maxChars) {
 // whatever happened to be last, which was always the footer.
 function clampLastLine(build, line, maxChars) {
   const footerText = directiveFooter({ mode: 'short' });
+  const bare = build([], footerText);
   // +1 for the newline the line itself brings when it joins the blocks.
-  const room = maxChars - build([], footerText).length - 1;
+  const room = maxChars - bare.length - 1;
   if (room >= 24) {
     const clipped = line.length > room ? `${line.slice(0, room - 1)}…` : line;
     return build([clipped], footerText);
   }
-  // maxChars is floored at 500 and header + short footer fit well inside
-  // that, so this is unreachable; keep the hard slice as the safety net.
-  return `${build([line], footerText).slice(0, maxChars - 1)}…`;
+  // No room for even a clipped finding line: the note reservation can pull
+  // the budget below the 500-char floor, and a deep file path can push the
+  // header past what remains beside the short policy (Bugbot on PR #508).
+  // Drop the line, and if the bare header + policy still overflow, clip the
+  // head. Never tail-slice: the footer sits at the end, so a tail slice is
+  // exactly the footer cut this renderer exists to prevent.
+  if (bare.length <= maxChars) return bare;
+  const head = bare.slice(0, Math.max(0, maxChars - footerText.length - 4));
+  return `${head}…\n\n${footerText}`;
 }
 
 // `compact` drops the registry description: within one emission the first

tests/hook.test.mjs

@@ -1224,6 +1224,24 @@ describe('renderTemplate()', () => {
     assert.match(renderFor('win32'), /ignore-value overused-font "Space Grotesk Var"/);
   });
 
+  it('keeps the policy footer when reserveChars presses against the 500-char floor', () => {
+    // Bugbot on PR #508: the note reservation used to be subtracted after
+    // the 500-char floor, so the clamp could run at ~366 chars, below the
+    // budget clampLastLine assumes safe, and the hard tail slice cut the
+    // policy footer. The reserve now comes off before the floor; when the
+    // floor wins, the note defers instead.
+    const config = { ...DEFAULT_CONFIG, limits: { ...DEFAULT_CONFIG.limits, maxChars: 500 } };
+    const longPath = `/x/${'deeply-nested/'.repeat(6)}Component.tsx`;
+    const text = renderTemplate(
+      Array.from({ length: 6 }, (_, i) =>
+        finding('side-tab', i + 1, { name: 'Side tab', description: 'Colored side border.' })),
+      longPath, config, { cwd: '/x', reserveChars: 134 }
+    );
+    assert.ok(text.length <= 500, `stays inside the floored budget (got ${text.length})`);
+    assert.match(text, /unsure, ask in one line\.$/);
+    assert.doesNotMatch(text, /…$/);
+  });
+
   it('drops the L<line> prefix when line is 0', () => {
     const text = renderTemplate(
       [finding('side-tab', 0, { name: 'X' })],