fix: add regression-test-presence gate + missing tests for #2429/#2279 (#2563)

lint-fix-has-regression-test.cjs: new gate that fails if a fix(#NNNN)
or feat(#NNNN) commit has zero behavioral test files (*.test.cjs,
excluding auto-generated fixtures/baselines) in its diff. Wired into
lint:ci so it runs before PR creation.

Missing regression tests added:
- #2429: codex local scope does not set $HOME/.agents skills home;
  global scope does (tests/runtime-artifact-layout.test.cjs)
- #2279: map-codebase instructions say to overwrite existing dates,
  not just replace [YYYY-MM-DD] placeholders (tests/commands.test.cjs)

scripts/lint-fix-has-regression-test.cjs

@@ -0,0 +1,131 @@
+#!/usr/bin/env node
+'use strict';
+
+/**
+ * lint-fix-has-regression-test.cjs — gate: every fix(#NNNN) commit must
+ * include at least one behavioral test file (tests/*.test.cjs) that is NOT
+ * an auto-generated fixture/baseline.
+ *
+ * ## Why
+ *
+ * CONTRIBUTING.md:47: "Fix it. Write a test that would have caught the bug."
+ * CLAUDE.md: "Regression Protocol: Write the regression test first."
+ *
+ * Three merged PRs shipped without regression tests because the golden-fixture
+ * regeneration (20+ files under tests/) camouflaged the absence of real
+ * *.test.cjs changes. This gate makes the rule machine-enforced.
+ *
+ * ## What this enforces
+ *
+ * For every commit in `git log origin/next..HEAD` whose subject matches
+ * /^fix\(#\d+\)/ or /^feat\(#\d+\)/, the cumulative diff must include at
+ * least one file under tests/ matching /\.test\.cjs$/ that is NOT:
+ *   - under tests/fixtures/
+ *   - under tests/install-tree/
+ *   - a *-baseline.json file
+ *
+ * If the filtered list is empty, the gate fails with a message naming the
+ * fix commits and directing the author to add a regression test.
+ *
+ * ## Overrides
+ *
+ * Set GSD_SKIP_REGRESSION_TEST_GATE=1 for legitimate exceptions (e.g. a
+ * pure-config or pure-refactor PR where no behavioral test is possible).
+ * This env var is auditable in CI logs.
+ */
+
+const { execSync } = require('child_process');
+const { ExitError, runMain } = require('./lib/cli-exit.cjs');
+
+const FIX_OR_FEAT_RE = /^(?:fix|feat)\(#(\d+)\)/;
+
+const EXCLUDE_PATTERNS = [
+  /tests[\\/]fixtures[\\/]/,
+  /tests[\\/]install-tree[\\/]/,
+  /-baseline\.json$/,
+];
+
+function isRealTestFile(filePath) {
+  if (!filePath.includes('tests/')) return false;
+  if (!/\.test\.cjs$/.test(filePath)) return false;
+  return !EXCLUDE_PATTERNS.some((re) => re.test(filePath));
+}
+
+function getFixCommits(baseRef) {
+  const log = execSync(
+    `git log ${baseRef}..HEAD --format='%H%x09%s' --no-merges`,
+    { encoding: 'utf8', timeout: 10000 },
+  );
+  return log
+    .trim()
+    .split('\n')
+    .filter(Boolean)
+    .map((line) => {
+      const [sha, subject] = line.split('\t');
+      const match = subject.match(FIX_OR_FEAT_RE);
+      return match ? { sha: sha.slice(0, 12), subject, issue: match[1] } : null;
+    })
+    .filter(Boolean);
+}
+
+function getChangedTestFiles(baseRef) {
+  const diff = execSync(
+    `git diff ${baseRef}...HEAD --name-only --no-merges`,
+    { encoding: 'utf8', timeout: 10000 },
+  );
+  return diff
+    .trim()
+    .split('\n')
+    .filter(Boolean)
+    .filter(isRealTestFile);
+}
+
+function main() {
+  if (process.env.GSD_SKIP_REGRESSION_TEST_GATE === '1') {
+    console.log('lint-fix-has-regression-test: SKIPPED (GSD_SKIP_REGRESSION_TEST_GATE=1)');
+    return;
+  }
+
+  const baseRef = process.env.GSD_REGRESSION_GATE_BASE || 'origin/next';
+
+  let fixCommits;
+  try {
+    fixCommits = getFixCommits(baseRef);
+  } catch {
+    console.log(`lint-fix-has-regression-test: no fix/feat commits found vs ${baseRef}, skipping`);
+    return;
+  }
+
+  if (fixCommits.length === 0) {
+    console.log('lint-fix-has-regression-test: no fix/feat commits, passing');
+    return;
+  }
+
+  let testFiles;
+  try {
+    testFiles = getChangedTestFiles(baseRef);
+  } catch {
+    testFiles = [];
+  }
+
+  if (testFiles.length === 0) {
+    const commitList = fixCommits
+      .map((c) => `  ${c.sha} ${c.subject}`)
+      .join('\n');
+    throw new ExitError(1,
+      `lint-fix-has-regression-test: ${fixCommits.length} fix/feat commit(s) but ZERO behavioral test files (*.test.cjs) in the diff.\n` +
+      `Auto-generated fixtures (tests/fixtures/, *-baseline.json) do NOT count.\n\n` +
+      `Fix commits:\n${commitList}\n\n` +
+      `CONTRIBUTING.md:47: "Write a test that would have caught the bug."\n` +
+      `Add a regression test to an existing tests/*.test.cjs file, or set ` +
+      `GSD_SKIP_REGRESSION_TEST_GATE=1 if no behavioral test is possible (auditable in CI).`
+    );
+  }
+
+  console.log(
+    `lint-fix-has-regression-test: PASS — ${fixCommits.length} fix/feat commit(s), ` +
+    `${testFiles.length} behavioral test file(s): ${testFiles.join(', ')}`
+  );
+}
+
+runMain(main);