Derive build version from git tags via VERSION file.

flake.nix reads from VERSION instead of hardcoding; Makefile gains
a version target that syncs VERSION from the latest git tag and
injects it into go build ldflags.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Makefile

@@ -1,4 +1,6 @@
-.PHONY: proto build test lint clean
+VERSION := $(shell git describe --tags --abbrev=0 | sed 's/^v//')
+
+.PHONY: proto build test lint clean version
 
 proto:
 	protoc \
@@ -7,8 +9,11 @@ proto:
 		-I proto \
 		proto/sgard/v1/sgard.proto
 
+version:
+	@echo $(VERSION) > VERSION
+
 build:
-	go build ./...
+	go build -ldflags "-X main.version=$(VERSION)" ./...
 
 test:
 	go test ./...