Derive the build version from the git tag

The version was a hand-edited constant in the Makefile, so tagging a release
without first committing the bump produced a release whose binaries disagree
with it: v0.2.1 was tagged on a commit still saying VERSION := v0.2.0, and
all four binaries attached to that release print v0.2.0 from `draft version`.

VERSION now comes from `git describe --tags --always`: exactly the tag on a
tagged commit (v0.2.1), tag-plus-distance after it (v0.2.1-3-g1a2b3c4), `dev`
outside a git checkout such as a source tarball. `make build VERSION=...`
still overrides it. There is nothing left to bump before tagging.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

Makefile

@@ -1,5 +1,9 @@
 PKG_NAME := git.fernandovideira.com/FernandoJVideira/draft
-VERSION := v0.2.0
+# Taken from the git tag, so a build can never disagree with the release it was
+# cut from: exactly `v0.2.1` on that tag, `v0.2.1-3-g1a2b3c4` three commits
+# later, `dev` outside a git checkout (e.g. a source tarball). Override with
+# `make build VERSION=v1.2.3`.
+VERSION ?= $(shell git describe --tags --always 2>/dev/null || echo dev)
 GO_SOURCES := $(shell find . -name '*.go' -not -path './assets/testdata/*')
 
 .PHONY: version build windows win-arm linux mac prepare-embed clean