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.

docs/system-architecture.md

@@ -49,7 +49,7 @@ FetchContributionsAllTime(ctx, profile, opts)
   │          TotalCommitsAllTime
   │
   ▼
-FetchProductive(ctx, profile, profile.SeedRepos, loc, commitsPerRepo)
+FetchProductive(ctx, profile, profile.SeedRepos, loc, commitsPerRepo)  // 0 = no cap
   │  commitHistoryQuery × (#seeds × pages)
   │  per commit: t = committedDate in loc
   │              ProductiveAllTime[t.Hour]++