Drop row-level label exclusion; make filtering per-chart explicit

Previously, --exclude-labels was applied at the SQL row level when
generating summaries — meaning a PR tagged 'no-changelog' was excluded
from EVERY chart (monthly trends, contributor heatmap, unique
contributors, etc.). That dropped most of pront's release/CI work from
the contributor charts (e.g. 39 real non-draft PRs in March 2026 became
3 after filtering).

The intent of --exclude-labels was always series-level: hide the
'no-changelog' bar from label-frequency charts where it would otherwise
dominate. Keep that, drop the row-level filter:

- Remove --exclude-labels CLI flag from generate-summaries and
  generate-all. The Rust side no longer filters rows by label.
- Keep --exclude-labels in plot.py for series-level hiding only.
- Expand the bot filter in plot.py to also drop logins stored without
  the '[bot]' suffix in older snapshots (dependabot, dependabot-preview,
  renovate, etc.). Otherwise removing the row filter pulls dependabot
  to the top of the contributor heatmap.
- Drop the global "labels excluded" note on trends pages; add a small
  italic note under each chart describing the exclusions that actually
  apply to it.

Heatmap polish (separate but bundled since it's regenerated anyway):
- Replace the YlOrRd colormap with a green→yellow→red gradient.
- Mask zero cells so empty months show the plain axes background
  instead of a yellow tint.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: pront

README.md

@@ -83,10 +83,10 @@ Writes to `out/historical/`. The fetched JSON must be split by year and promoted
 
 ```shell
 github-tools generate-all \
-  --env-file vector.env --env-file vrl.env --env-file quickwit.env \
-  --exclude-labels "no-changelog,meta: awaiting author"
+  --env-file vector.env --env-file vrl.env --env-file quickwit.env
 
-# Charts (still Python):
+# Charts (still Python). --exclude-labels hides those label series from
+# label-frequency charts only; the underlying PR/issue counts are unaffected.
 python -m scripts.util.plot --env-file vector.env --input-dir out/summaries \
   --start $(date -d "$(date +%Y-%m-01) -12 months" +%Y-%m) \
   --exclude-labels "no-changelog,meta: awaiting author"
@@ -111,5 +111,4 @@ Per-repo trend pages with all charts:
 - [VRL](trends/vrl.md)
 - [Quickwit](trends/quickwit.md)
 
-> [!NOTE]
-> Issues and PRs with the following labels are excluded from all charts: `no-changelog`, `meta: awaiting author`.
+Exclusions are now per-chart; see the note below each chart on the trends pages.