Weekly-pipeline cleanup: remove deprecated workflow and follow-ups

[BenjSz]

Summary

• Remove deprecated workflow. Delete `.github/workflows/test_weekly_DEPRECATED.yaml` — a Saturday-scheduled job that just printed the top-10 embedding-model report to logs. Its purpose has been fully absorbed by the ClickHouse-backed pipeline in `push-to-clickhouse.yaml` + `tests/spyre/weekly_generation/`.
• Rename `generate-matrix` → `generate-shards` in `push-to-clickhouse.yaml`. That job's real output is a set of shard files uploaded as an artifact; the `matrix_x{1,2,4}` JSON it emits is just an index into that artifact. The old name confusingly overlapped with the actual matrix-generator jobs of the same name in `test_daily.yaml` / `_test_matrix.yaml`.
• Clean up docstrings in `push-to-clickhouse.yaml`: rewrite the top-of-file overview to describe what the fetch/shard/fan-out flow actually does, and drop the stale reference to the deleted `test_weekly_DEPRECATED.yaml`.
• Accept `DD/MM/YYYY` dates in `tests/spyre/weekly_generation/clickhouse_db.py`. `import_csv` used to reject rows whose `snapshot_date`/`added_date` came from a European-formatted CSV export. A new shared `_parse_date` helper accepts ISO first, then a few day-first fallbacks — ambiguous US month-first strings are intentionally not accepted.
• Drop dead plot code in `tests/spyre/weekly_generation/snapshot_model_counts.py`: the two commented-out `_plot_1`/`_plot_2` blocks and the disabled funnel-bar section are gone. `_plot_3` is now the only plot function, renamed to `_plot_models`, and its "Three-panel" docstring corrected to "Two-panel".

Test plan

• `gh workflow list` no longer shows the deprecated `test_weekly` workflow.
• Confirm no other workflow references the deleted file (verified locally: `git grep test_weekly_DEPRECATED` returns nothing).
• Manually dispatch `push-to-clickhouse.yaml` with a small `top_k` to confirm the `generate-shards` rename didn't break the `needs:` wiring for the three `weekly-model-scan-x{1,2,4}` jobs.
• Run `snapshot_model_counts.py` against an enriched CSV to confirm the plot still renders after the `_plot_3 → _plot_models` rename.
• Run `clickhouse_db.py --add_csv` against a CSV containing a `DD/MM/YYYY` `snapshot_date` and confirm the row imports without hitting the "invalid snapshot_date" branch.

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