Filter commits by date

## Goal

When a user selects a time range, the visualization should reflect the actual commit count within that window — not just whether activity overlapped. Link widths, contributor node sizes, and flow visuals should all scale to the filtered commit counts. This also lays the groundwork for a future animation feature that steps through time.

## Implementation Order

1. **Python model change** — add `commit_histogram` field to Link
2. **Python CLI change** — bucket commits by month during `Link.from_github()`
3. **CSV generation** — write `commit_activity.csv` in the `csvs` command
4. **Build command** — copy new CSV to `assets/data/`
5. **Re-run data collection** — `python -m contributor_network data` then `csvs` then `build`
6. **JS data loading** — load fourth CSV, build activity index
7. **JS data preparation** — attach histograms to link objects
8. **Time range filtering** — implement count-based filtering in `applyFilters()`
9. **Scale updates** — verify link widths and contributor radii update correctly
10. **UI slider** — build the time range control

## Risks and Considerations

**API rate limits:** No additional API calls are needed. The commit data is already being fetched — we're just retaining the dates instead of discarding them.

**Data freshness:** Existing JSON files in `links/` won't have `commit_histogram` until re-fetched. The field defaults to `{}` so old data won't break, but will produce empty histograms. A full re-fetch is needed for complete temporal data.

**Month boundary precision:** Commits are bucketed by calendar month (UTC). A commit at 11:59pm on Jan 31 and one at 12:01am on Feb 1 land in different buckets. This is acceptable for the visualization's granularity.

**Scale behavior:** When the time range is very narrow (e.g., one month), most links will have small counts and the scale domain shrinks. This can make thin links appear thick. Consider setting a minimum domain ceiling (e.g., 10) to prevent scale distortion on narrow ranges.

**Backward compatibility:** `links.csv` is unchanged. The new `commit_activity.csv` is additive. If the JS can't find it, fall back to the overlap-based filtering described in the ROADMAP.md Feature 4 entry.