fix: add missing v3 release script and changelog template

The nightly-release-v3.yml workflow was failing because it references
v3/tasks/release/release.go which did not exist. This adds:

- v3/tasks/release/release.go: Release script with --check-only and
  --dry-run flags for the nightly release workflow
- v3/UNRELEASED_CHANGELOG.md: Template file for tracking unreleased
  changes

https://claude.ai/code/session_016FpJHv9CBS7YG9Mj6QJ7xd

v3/UNRELEASED_CHANGELOG.md

@@ -0,0 +1,20 @@
+# Unreleased Changelog
+
+All notable changes to the v3 alpha will be documented in this file before release.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## Categories
+
+Use the following categories to organize your entries:
+- `Added` for new features.
+- `Changed` for changes in existing functionality.
+- `Deprecated` for soon-to-be removed features.
+- `Removed` for now removed features.
+- `Fixed` for any bug fixes.
+- `Security` in case of vulnerabilities.
+
+---
+
+<!-- Add your changelog entries below this line -->
+

v3/tasks/release/release.go

@@ -0,0 +1,247 @@
+package main
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+)
+
+const (
+	unreleasedChangelogFile = "../../UNRELEASED_CHANGELOG.md"
+	versionPrefix           = "v3.0.0-alpha."
+)
+
+var (
+	checkOnly = flag.Bool("check-only", false, "Only check if there's unreleased content, exit 0 if yes, 1 if no")
+	dryRun    = flag.Bool("dry-run", false, "Run in dry-run mode (no actual release)")
+)
+
+func main() {
+	flag.Parse()
+
+	// Check for unreleased content
+	hasContent, entries := checkUnreleasedContent()
+
+	if *checkOnly {
+		if hasContent {
+			fmt.Println("Found unreleased changelog content")
+			os.Exit(0)
+		} else {
+			fmt.Println("No unreleased changelog content found")
+			os.Exit(1)
+		}
+	}
+
+	if !hasContent {
+		fmt.Println("No unreleased changelog content found. Nothing to release.")
+		os.Exit(0)
+	}
+
+	// Determine the next version
+	nextVersion := getNextVersion()
+	releaseTag := versionPrefix + strconv.Itoa(nextVersion)
+
+	fmt.Printf("Preparing release: %s\n", releaseTag)
+
+	if *dryRun {
+		fmt.Println("[DRY-RUN] Would perform the following actions:")
+		fmt.Printf("[DRY-RUN] - Create release tag: %s\n", releaseTag)
+		fmt.Printf("[DRY-RUN] - Changelog entries to include:\n")
+		for _, entry := range entries {
+			fmt.Printf("[DRY-RUN]   %s\n", entry)
+		}
+		setGitHubOutput("release_version", releaseTag)
+		setGitHubOutput("release_tag", releaseTag)
+		setGitHubOutput("release_dry_run", "true")
+		fmt.Println("[DRY-RUN] Release simulation complete")
+		return
+	}
+
+	// Perform actual release
+	if err := performRelease(releaseTag, entries); err != nil {
+		fmt.Printf("Release failed: %v\n", err)
+		os.Exit(1)
+	}
+
+	// Set GitHub outputs
+	setGitHubOutput("release_version", releaseTag)
+	setGitHubOutput("release_tag", releaseTag)
+	setGitHubOutput("release_dry_run", "false")
+
+	fmt.Printf("Release %s completed successfully!\n", releaseTag)
+}
+
+func checkUnreleasedContent() (bool, []string) {
+	content, err := os.ReadFile(unreleasedChangelogFile)
+	if err != nil {
+		fmt.Printf("Warning: Could not read %s: %v\n", unreleasedChangelogFile, err)
+		return false, nil
+	}
+
+	lines := strings.Split(string(content), "\n")
+	var entries []string
+
+	// Look for bullet point entries only after the separator line (---)
+	// This avoids matching the template category descriptions
+	foundSeparator := false
+	bulletRegex := regexp.MustCompile(`^\s*-\s+[^\s]`)
+
+	for _, line := range lines {
+		// Look for the separator line
+		if strings.TrimSpace(line) == "---" {
+			foundSeparator = true
+			continue
+		}
+
+		// Only match entries after the separator
+		if foundSeparator && bulletRegex.MatchString(line) {
+			entries = append(entries, strings.TrimSpace(line))
+		}
+	}
+
+	return len(entries) > 0, entries
+}
+
+func getNextVersion() int {
+	// Get all existing v3.0.0-alpha.* tags
+	cmd := exec.Command("git", "tag", "--list", "v3.0.0-alpha.*")
+	output, err := cmd.Output()
+	if err != nil {
+		fmt.Println("No existing alpha tags found, starting with alpha.1")
+		return 1
+	}
+
+	tags := strings.Split(strings.TrimSpace(string(output)), "\n")
+	if len(tags) == 0 || (len(tags) == 1 && tags[0] == "") {
+		return 1
+	}
+
+	// Extract version numbers and find the highest
+	var versions []int
+	for _, tag := range tags {
+		tag = strings.TrimSpace(tag)
+		if tag == "" {
+			continue
+		}
+		// Extract the number after "v3.0.0-alpha."
+		parts := strings.Split(tag, "v3.0.0-alpha.")
+		if len(parts) == 2 {
+			num, err := strconv.Atoi(parts[1])
+			if err == nil {
+				versions = append(versions, num)
+			}
+		}
+	}
+
+	if len(versions) == 0 {
+		return 1
+	}
+
+	sort.Ints(versions)
+	return versions[len(versions)-1] + 1
+}
+
+func performRelease(releaseTag string, entries []string) error {
+	// Reset the unreleased changelog
+	if err := resetUnreleasedChangelog(); err != nil {
+		return fmt.Errorf("failed to reset unreleased changelog: %w", err)
+	}
+
+	// Stage the changes
+	if err := runCommand("git", "add", unreleasedChangelogFile); err != nil {
+		return fmt.Errorf("failed to stage changelog: %w", err)
+	}
+
+	// Create commit
+	commitMsg := fmt.Sprintf("chore: release %s", releaseTag)
+	if err := runCommand("git", "commit", "-m", commitMsg); err != nil {
+		return fmt.Errorf("failed to create commit: %w", err)
+	}
+
+	// Create annotated tag
+	tagMsg := fmt.Sprintf("Release %s\n\nChanges:\n%s", releaseTag, strings.Join(entries, "\n"))
+	if err := runCommand("git", "tag", "-a", releaseTag, "-m", tagMsg); err != nil {
+		return fmt.Errorf("failed to create tag: %w", err)
+	}
+
+	// Push commit and tag
+	if err := runCommand("git", "push", "origin", "HEAD"); err != nil {
+		return fmt.Errorf("failed to push commit: %w", err)
+	}
+
+	if err := runCommand("git", "push", "origin", releaseTag); err != nil {
+		return fmt.Errorf("failed to push tag: %w", err)
+	}
+
+	return nil
+}
+
+func resetUnreleasedChangelog() error {
+	template := `# Unreleased Changelog
+
+All notable changes to the v3 alpha will be documented in this file before release.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## Categories
+
+Use the following categories to organize your entries:
+- ` + "`Added`" + ` for new features.
+- ` + "`Changed`" + ` for changes in existing functionality.
+- ` + "`Deprecated`" + ` for soon-to-be removed features.
+- ` + "`Removed`" + ` for now removed features.
+- ` + "`Fixed`" + ` for any bug fixes.
+- ` + "`Security`" + ` in case of vulnerabilities.
+
+---
+
+<!-- Add your changelog entries below this line -->
+
+`
+	return os.WriteFile(unreleasedChangelogFile, []byte(template), 0644)
+}
+
+func runCommand(name string, args ...string) error {
+	cmd := exec.Command(name, args...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
+}
+
+func setGitHubOutput(name, value string) {
+	// Write to GITHUB_OUTPUT file if available (GitHub Actions)
+	outputFile := os.Getenv("GITHUB_OUTPUT")
+	if outputFile != "" {
+		f, err := os.OpenFile(outputFile, os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0644)
+		if err == nil {
+			defer f.Close()
+			writer := bufio.NewWriter(f)
+			fmt.Fprintf(writer, "%s=%s\n", name, value)
+			writer.Flush()
+		}
+	}
+	// Also print for visibility
+	fmt.Printf("::set-output name=%s::%s\n", name, value)
+}
+
+// getToday returns today's date in YYYY-MM-DD format (used for release notes)
+func getToday() string {
+	return time.Now().Format("2006-01-02")
+}
+
+// getReleaseDir returns the directory where release artifacts should be stored
+func getReleaseDir() string {
+	dir, err := filepath.Abs(".")
+	if err != nil {
+		return "."
+	}
+	return dir
+}