Add activity_count.py, which walks both git histories and records, per
month since 2021: non-merge commits, active contributors and cumulative
distinct contributors. Authors are deduplicated via mailmap and bots are
excluded. The cumulative count covers each project's full history, not
just the reported range.
activity-graph.py renders the three metrics as stacked panels sharing an
x-axis, in the same style as the other graphs.
The CI job checks out both histories (blobless, full depth), regenerates
the data and commits the refreshed graph, like the other trackers.
Also fix the lint errors the latest ruff reports on the existing files,
which were failing the lint job.
