Report development builds as such, not as the release version

Only a release build (the commit tagged v<VERSION>, unchanged) gives a
version number. Others print 'dev <commit> (3 commits after v0.2.0-rc.1)',
from git describe (the Makefile's DESCRIBE), in jarld version,
jarl --version, jarl status, jarl whoami and the startup log; a build
without git says '(source build)'. The CLI's User-Agent and the footer's
tooltip follow suit, and /status's build gains since and ahead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Makefile

@@ -18,7 +18,11 @@ VERSION ?= $(shell cat VERSION 2>/dev/null || echo 0.0.0-dev)
 # the release rather than the commit): set when HEAD is tagged v$(VERSION)
 # and no tracked file is changed.
 RELEASE_TAG ?= $(shell t=v$(VERSION); git tag --points-at HEAD 2>/dev/null | grep -qx "$$t" && git diff --quiet HEAD 2>/dev/null && echo $$t)
-LDFLAGS := -s -w -X jarl/internal/version.Version=$(VERSION) -X jarl/internal/version.Tag=$(RELEASE_TAG)
+# DESCRIBE places other builds relative to the last tag ("3 commits after
+# v0.2.0-rc.1").
+DESCRIBE ?= $(shell git describe --tags --long --match 'v[0-9]*' --abbrev=12 HEAD 2>/dev/null)
+LDFLAGS := -s -w -X jarl/internal/version.Version=$(VERSION) -X jarl/internal/version.Tag=$(RELEASE_TAG) \
+	-X jarl/internal/version.Describe=$(DESCRIBE)
 GOFLAGS := -trimpath
 export CGO_ENABLED := 0