docs: ship the leakage chart renderer as a public script

[shivamlalakiya]

Why

r/dataisbeautiful requires a comment linking the tool and the data source. The renderer that produced the leakage figure lived in scripts/announce/, which is gitignored, so there was no public link to give.

What

• scripts/render_leakage_chart.py, sitting next to the two leakage experiments it re-runs. It calls the same _panel / _clf helpers and the same FiscalYearGroupedSplitter walk-forward CV as leakage_experiment.py and real_data_leakage_experiment.py, but keeps the per-fold scores instead of averaging them, so the figure shows the shape of the backtest rather than one number per condition.
• --cached re-plots from a saved score file (the real panel takes ~9 minutes to refit), --out picks the output directory.
• .gitignore: scripts/announce/ and the other local working-note paths are now ignored explicitly rather than by accident, and the rendered figure files stay out of the tree.
• CHANGELOG entry under ## [Unreleased].

No library code changes; nothing under philanthropy/ is touched.

Verification

Re-plotted from the committed run's score cache and diffed the numbers against docs/explanation/benchmarks.md and the preprint:

$ python scripts/render_leakage_chart.py --cached --out /tmp/chart
synthetic: as_of_mean 0.6248, whole_mean 0.7504
real:      as_of_mean 0.4820, whole_mean 0.8578

Matches the published 0.625 / 0.750 and 0.482 / 0.858.