The changelog engine: turn git diffs into a civic changelog

The diff is the product — now literally. generate_changelog.py reads a
git diff of the entity tree between any two refs and says, in plain
language, what changed in the government: entities added / modified /
removed, grouped by type and by jurisdiction, dated from the commit.

CHANGELOG.md is seeded from the real history so far:
  - initial load: +11,285 people across 51 states
  - institutional gap: +233 bodies, ~532 people enriched
  - elections + demographics: +2,494 candidates, +3,494 jurisdictions
    (TX +541, FL +396, CA +341 ...)

On an initial load every entity is an addition; on a re-sync of the same
source, only real government changes will surface — which is the whole
point. When nothing changed, it says so.

make changelog [BASE=.. HEAD=..]. No third-party deps.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Makefile

@@ -9,4 +9,7 @@ build:        ## Regenerate the entity tree from raw exports in data/
 validate:     ## Check every entity file: OKF, schema, and link integrity
 	python3 scripts/validate.py
 
+changelog:    ## What changed in the government since the last commit (BASE/HEAD overridable)
+	python3 scripts/generate_changelog.py $(BASE) $(HEAD)
+
 check: build validate  ## Build, then validate — the full gate