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>

internal/version/version_test.go

@@ -20,3 +20,30 @@ func TestDescribe(t *testing.T) {
 		}
 	}
 }
+
+func TestString(t *testing.T) {
+	const c = "ca0f148b78b6e1f0a9b8c7d6e5f4a3b2c1d0e9f8"
+	build := func(tag, commit string, modified bool, desc string) Build {
+		b := describe("0.2.0-rc.1", tag, commit, modified)
+		b.Since, b.Ahead = parseDescribe(desc, b.Commit)
+		return b
+	}
+	for _, tc := range []struct {
+		b    Build
+		want string
+	}{
+		{build("v0.2.0-rc.1", c, false, "v0.2.0-rc.1-0-gca0f148b78b6"), "0.2.0-rc.1"},
+		{build("", c, false, "v0.2.0-rc.1-3-gca0f148b78b6"), "dev ca0f148b78b6 (3 commits after v0.2.0-rc.1)"},
+		{build("", c, true, "v0.2.0-rc.1-1-gca0f148b78b6"), "dev ca0f148b78b6-modified (1 commit after v0.2.0-rc.1)"},
+		{build("", c, true, "v0.2.0-rc.1-0-gca0f148b78b6"), "dev ca0f148b78b6-modified (v0.2.0-rc.1 with uncommitted changes)"},
+		{build("", c, false, "v0.1.0-0-gca0f148b78b6"), "dev ca0f148b78b6 (at v0.1.0)"},
+		{build("", c, false, ""), "dev ca0f148b78b6"},
+		// A describe of another commit (stale ldflags) is ignored.
+		{build("", c, false, "v0.2.0-rc.1-3-gdeadbeef0000"), "dev ca0f148b78b6"},
+		{build("", "", false, ""), "0.2.0-rc.1 (source build)"},
+	} {
+		if got := tc.b.String(); got != tc.want {
+			t.Errorf("String() = %q, want %q", got, tc.want)
+		}
+	}
+}