Docs: document the Board pipeline + Artifact redeploy step

Add `make board` and docs/board.md so the Board's rebuild-and-redeploy flow
is a documented step, not tribal knowledge. Covers why the county data is
inlined (self-contained Artifact / CSP), that build_viz.py injects it into
board.html, and that a live Artifact must be REPUBLISHED (editing the repo
file doesn't update a deployed Artifact). README links it in both the make
list and the docs index.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Makefile

@@ -1,7 +1,7 @@
 # Republic OS — repeatable pipeline commands.
 # The build is deterministic: `make build` twice yields a byte-identical tree.
 
-.PHONY: build validate legal-us-code check
+.PHONY: build validate board legal-us-code check
 
 build:        ## Regenerate the entity tree from raw exports in data/
 	python3 scripts/build.py
@@ -9,6 +9,13 @@ build:        ## Regenerate the entity tree from raw exports in data/
 validate:     ## Check every entity file: OKF, schema, and link integrity
 	python3 scripts/validate.py
 
+board:        ## Rebuild the Board (inlines fresh data into viz/board.html)
+	python3 scripts/build_viz.py
+	@echo
+	@echo "board.html regenerated. To update the LIVE Artifact you must REPUBLISH"
+	@echo "viz/board.html to its existing URL — editing the repo file does not"
+	@echo "update a deployed Artifact. See docs/board.md."
+
 changelog:    ## What changed in the government since the last commit (BASE/HEAD overridable)
 	python3 scripts/generate_changelog.py $(BASE) $(HEAD)