test: delete duplicate-coverage tests, fix TESTS.md drift, drop hand-rolled itoa

Items 1-3 of the 2026-08-04 test-suite review: TestCleanupLogsKeepsFilesWithinAgeLimit,
TestRunDueEmptyOverlapInheritsGlobal, and TestSameWindowsPathHandlesSpaces had
byte-identical coverage to an existing test and no assertion the survivor lacked.
storage.defaultJobs, the one accidental 0% coverage gap the review found, is now
covered and TESTS.md corrected to match. seed_test.go's itoa is replaced with
strconv.FormatInt.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/runner/seed_test.go

@@ -22,7 +22,7 @@ func writeTestLog(t *testing.T, dir, filename, state string, durationMS int64, j
 		content.WriteString("\n")
 	}
 	if durationMS >= 0 {
-		content.WriteString("state: " + state + "\nduration: " + itoa(durationMS) + "\n\n")
+		content.WriteString("state: " + state + "\nduration: " + strconv.FormatInt(durationMS, 10) + "\n\n")
 	} else {
 		content.WriteString("state: " + state + "\n\n")
 	}
@@ -31,25 +31,6 @@ func writeTestLog(t *testing.T, dir, filename, state string, durationMS int64, j
 	}
 }
 
-func itoa(n int64) string {
-	if n == 0 {
-		return "0"
-	}
-	neg := n < 0
-	if neg {
-		n = -n
-	}
-	buf := make([]byte, 0, 20)
-	for n > 0 {
-		buf = append([]byte{byte('0' + n%10)}, buf...)
-		n /= 10
-	}
-	if neg {
-		buf = append([]byte{'-'}, buf...)
-	}
-	return string(buf)
-}
-
 func TestSeedStatsBasic(t *testing.T) {
 	dir := t.TempDir()
 	job := domain.Job{ID: 1, Name: "Build"}