chore(docs): replace d2 diagram pipeline with mermaid

Remove docs/_diagrams/ and d2 build step from Makefile. The HTML
template already initialises mermaid.js; diagrams are now inline
code fences rendered client-side.

Makefile

@@ -14,8 +14,6 @@ SANDERLING_BIN := bin/sanderling
 DOCS_SRC := $(shell find docs -type f -name '*.md' -not -path 'docs/_*')
 DOCS_OUT := $(patsubst docs/%.md,build/site/%.html,$(DOCS_SRC))
 DOCS_TEMPLATE := docs/_template/page.html
-DIAGRAM_SRC := $(shell find docs/_diagrams -type f -name '*.d2' 2>/dev/null)
-DIAGRAM_OUT := $(patsubst docs/_diagrams/%.d2,build/site/_assets/diagrams/%.svg,$(DIAGRAM_SRC))
 
 INSPECT_DIST := internal/inspect/dist
 WEB_DIST := inspect-ui/dist
@@ -84,18 +82,14 @@ test-kotlin:
 test-spec-api:
 	cd pkg/spec && npm test --silent
 
-docs: $(DOCS_OUT) build/site/_assets $(DIAGRAM_OUT)
-	@echo "built $(words $(DOCS_OUT)) pages, $(words $(DIAGRAM_OUT)) diagrams to build/site"
+docs: $(DOCS_OUT) build/site/_assets
+	@echo "built $(words $(DOCS_OUT)) pages to build/site"
 
 build/site/_assets: docs/_assets
 	@mkdir -p build/site
 	@rm -rf $@
 	@cp -R $< $@
 
-build/site/_assets/diagrams/%.svg: docs/_diagrams/%.d2
-	@mkdir -p $(dir $@)
-	@d2 --theme 301 --pad 20 $< $@
-
 build/site/%.html: docs/%.md $(DOCS_TEMPLATE)
 	@mkdir -p $(dir $@)
 	@pandoc $< --from=gfm --to=html5 --standalone \