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.go

@@ -4,12 +4,17 @@
 //
 // The Makefile sets Version from the VERSION file, and Tag only when the
 // build is of that release: HEAD is the tag v<VERSION> and no tracked file
-// is changed. Other builds are identified by their commit, which Go
-// records from git by itself.
+// is changed. Only such a build reports a version number. Other builds
+// are development builds, identified by their commit (which Go records
+// from git by itself) and by where they stand relative to the last tag
+// (Describe, from git describe).
 package version
 
 import (
+	"regexp"
 	"runtime/debug"
+	"strconv"
+	"strings"
 	"sync"
 )
 
@@ -23,6 +28,11 @@ var Tag = ""
 // information when not set at build time.
 var Commit = ""
 
+// Describe is 'git describe --tags --long' of the build's commit
+// ("v0.2.0-rc.1-3-gca0f148b78b6"), set by the Makefile; it says how many
+// commits a development build is past the last tag.
+var Describe = ""
+
 // Repo is Jarl's source repository.
 const Repo = "https://gitea.adbraun.lol/adam/jarl"
 
@@ -32,6 +42,11 @@ type Build struct {
 	Tag      string `json:"tag,omitempty"`
 	Commit   string `json:"commit,omitempty"`
 	Modified bool   `json:"modified,omitempty"` // built with uncommitted changes
+	// Since is the last tag before a development build's commit, and
+	// Ahead how many commits the build is past it (0: the tag's commit,
+	// with uncommitted changes).
+	Since string `json:"since,omitempty"`
+	Ahead int    `json:"ahead,omitempty"`
 	// Label is the short form to show: the tag of a release, otherwise
 	// the abbreviated commit (or the version when the commit is unknown).
 	Label string `json:"label"`
@@ -53,9 +68,23 @@ var info = sync.OnceValue(func() Build {
 			}
 		}
 	}
-	return describe(Version, Tag, commit, modified)
+	b := describe(Version, Tag, commit, modified)
+	b.Since, b.Ahead = parseDescribe(Describe, b.Commit)
+	return b
 })
 
+// parseDescribe splits git describe --long output, if it is of commit.
+func parseDescribe(d, commit string) (since string, ahead int) {
+	m := describeRe.FindStringSubmatch(d)
+	if m == nil || commit == "" || !strings.HasPrefix(commit, m[3]) {
+		return "", 0
+	}
+	n, _ := strconv.Atoi(m[2])
+	return m[1], n
+}
+
+var describeRe = regexp.MustCompile(`^(v.+)-(\d+)-g([0-9a-f]+)$`)
+
 func describe(version, tag, commit string, modified bool) Build {
 	b := Build{Version: version, Tag: tag, Commit: commit, Modified: modified}
 	switch {
@@ -76,14 +105,41 @@ func describe(version, tag, commit string, modified bool) Build {
 // Info describes this build.
 func Info() Build { return info() }
 
-// String is the version for logs and 'version' commands: "0.2.0" for a
-// release, otherwise the version and the commit ("0.2.0-dev (a1b2c3d4e5f6)").
-func String() string {
-	b := Info()
-	if b.Tag != "" || b.Commit == "" {
+// String is the version for logs and 'version' commands. Only a release
+// build gives a version ("0.2.0"); any other build says it's a
+// development build: "dev a1b2c3d4e5f6 (3 commits after v0.2.0-rc.1)".
+func String() string { return Info().String() }
+
+func (b Build) String() string {
+	switch {
+	case b.Tag != "":
 		return b.Version
+	case b.Commit == "":
+		// No git information (built from a source archive).
+		return b.Version + " (source build)"
 	}
-	return b.Version + " (" + b.Label + ")"
+	s := "dev " + b.Label
+	switch {
+	case b.Since == "":
+	case b.Ahead == 1:
+		s += " (1 commit after " + b.Since + ")"
+	case b.Ahead > 1:
+		s += " (" + strconv.Itoa(b.Ahead) + " commits after " + b.Since + ")"
+	case b.Modified:
+		s += " (" + b.Since + " with uncommitted changes)"
+	default:
+		s += " (at " + b.Since + ")"
+	}
+	return s
+}
+
+// Short is the version for a User-Agent: the version of a release,
+// otherwise the label ("a1b2c3d4e5f6").
+func Short() string {
+	if b := Info(); b.Tag == "" && b.Commit != "" {
+		return b.Label
+	}
+	return Version
 }
 
 func short(commit string) string {