Add --version flag to print build info (#411)

## Summary

Adds a `--version` flag to the genqlient CLI that displays version
information including commit hash and build date.

## Changes

- Added `--version` flag to `generate/cliArgs` struct
- Implemented `printVersion()` function that uses
`runtime/debug.BuildInfo` to extract:
  - Module version (or "dev" for development builds)
  - Git commit hash (shortened to 12 characters)
  - Build timestamp
- Updated changelog with new feature entry

## Example Output

For development builds:
```
genqlient dev (8ddeeee, built 2025-10-28T05:47:17Z)
```

For release builds:
```
genqlient v1.2.3 (abc123def456, built 2025-10-28T05:47:17Z)
```

## Testing

- ✅ All existing tests pass
- ✅ Lint passes (`make lint`)
- ✅ Manually tested with `go run . --version` and `go build` +
`./genqlient --version`
- ✅ Help output correctly shows the new flag

## Notes

The version flag follows common CLI conventions and provides useful
debugging information. Pseudo-versions (like `v0.0.0-...`) are
normalized to "dev" for cleaner output, while real version tags display
normally.

Issue #296

I have:
- [x] Written a clear PR title and description (above)
- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
- [x] Added tests covering my changes, if applicable
- [x] Included a link to the issue fixed, if applicable
- [x] Included documentation, for new features
- [x] Added an entry to the changelog

Author: irubey

docs/CHANGELOG.md

@@ -28,6 +28,8 @@ Note that genqlient now requires Go 1.23 or higher, and is tested through Go 1.2
 
 ### New features:
 
+- Added `--version` flag to print version information including commit hash and build date
+
 ### Bug fixes:
 
 - fixed minor typos and grammatical issues across the project

generate/main.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime/debug"
 	"strings"
 
 	"github.com/alexflint/go-arg"
@@ -59,6 +60,7 @@ func readConfigGenerateAndWrite(configFilename string) error {
 type cliArgs struct {
 	ConfigFilename string `arg:"positional" placeholder:"CONFIG" default:"" help:"path to genqlient configuration (default: genqlient.yaml in current or any parent directory)"`
 	Init           bool   `arg:"--init" help:"write out and use a default config file"`
+	Version        bool   `arg:"--version" help:"print version information"`
 }
 
 func (cliArgs) Description() string {
@@ -68,6 +70,42 @@ See https://github.com/Khan/genqlient for full documentation.
 `)
 }
 
+func printVersion() {
+	info, ok := debug.ReadBuildInfo()
+	if !ok {
+		fmt.Println("genqlient (version information not available)")
+		return
+	}
+
+	version := info.Main.Version
+	if version == "" || version == "(devel)" || strings.HasPrefix(version, "v0.0.0-") {
+		version = "dev"
+	}
+
+	var commit, buildDate string
+	for _, setting := range info.Settings {
+		switch setting.Key {
+		case "vcs.revision":
+			commit = setting.Value
+		case "vcs.time":
+			buildDate = setting.Value
+		}
+	}
+
+	fmt.Print("genqlient " + version)
+	if commit != "" {
+		if len(commit) > 12 {
+			commit = commit[:12]
+		}
+		fmt.Printf(" (%s", commit)
+		if buildDate != "" {
+			fmt.Printf(", built %s", buildDate)
+		}
+		fmt.Print(")")
+	}
+	fmt.Println()
+}
+
 // Main is the command-line entrypoint to genqlient; it's equivalent to calling
 //
 //	go run github.com/Khan/genqlient
@@ -83,6 +121,12 @@ func Main() {
 
 	var args cliArgs
 	arg.MustParse(&args)
+
+	if args.Version {
+		printVersion()
+		return
+	}
+
 	if args.Init {
 		filename := args.ConfigFilename
 		if filename == "" {