feat: let commits-per-repo 0 mean every commit

The pagination guard broke on seen >= maxPerRepo, so a cap of 0 stopped
before the first page and rendered empty productive-time,
productive-weekday and most-commit-language cards. That inverted the
meaning 0 carries for top-repos, where it already means unlimited.

Treat a cap of 0 or less as no cap. Verified against a real repo: a cap
of 100 still stops at 100, while 500 and 0 both walk the full 382-commit
history.

internal/github/commit_cap_test.go

@@ -0,0 +1,27 @@
+package github
+
+import "testing"
+
+func TestReachedCommitCap(t *testing.T) {
+	cases := []struct {
+		name       string
+		seen       int
+		maxPerRepo int
+		want       bool
+	}{
+		{"under the cap", 100, 500, false},
+		{"at the cap", 500, 500, true},
+		{"past the cap", 700, 500, true},
+		// Zero means unlimited, so even a fresh repo keeps paginating. The
+		// old behavior stopped here and rendered empty commit-derived cards.
+		{"zero cap, nothing seen yet", 0, 0, false},
+		{"zero cap, deep into history", 100_000, 0, false},
+		{"negative cap treated as unlimited", 10, -1, false},
+	}
+	for _, tc := range cases {
+		if got := reachedCommitCap(tc.seen, tc.maxPerRepo); got != tc.want {
+			t.Errorf("%s: reachedCommitCap(%d, %d) = %v, want %v",
+				tc.name, tc.seen, tc.maxPerRepo, got, tc.want)
+		}
+	}
+}