feat: initial stamp implementation

- stamp add: create .stamp/*.md changeset files (interactive + --no-interactive)
- stamp status: show pending stamps and projected next versions (plain + --json)
- stamp version: consume stamps, bump semver, update CHANGELOGs, commit
- stamp publish: create git tags, GitHub/Gitea releases, upload artifacts
- stamp comment: post/update idempotent PR comments via GitHub/Gitea API
- Supports monorepos via stamp.toml [[projects]] blocks
- Changeset files support YAML (---) and TOML (+++) frontmatter
- Semver + pre-release support (alpha, beta, rc)
- Examples and workflow documentation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/publish/publish.go

@@ -0,0 +1,141 @@
+package publish
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/thokra/stamp/internal/config"
+	"github.com/thokra/stamp/internal/git"
+	ghClient "github.com/thokra/stamp/internal/github"
+	giteaClient "github.com/thokra/stamp/internal/gitea"
+)
+
+// Options controls publish behaviour.
+type Options struct {
+	DryRun   bool
+	RepoSlug string // "owner/repo" — required for releases
+	RepoRoot string
+}
+
+// Publish creates tags, releases, and uploads artifacts for a project.
+func Publish(cfg *config.Config, project *config.Project, opts Options) error {
+	tag := git.TagName(project.Name, project.Version)
+
+	// --- Git tag ---
+	if project.Publish.PublishTags() {
+		if opts.DryRun {
+			fmt.Printf("[dry-run] would create git tag %s\n", tag)
+		} else {
+			exists, err := git.TagExists(opts.RepoRoot, tag)
+			if err != nil {
+				return err
+			}
+			if exists {
+				fmt.Printf("tag %s already exists, skipping\n", tag)
+			} else {
+				if err := git.Tag(opts.RepoRoot, tag, fmt.Sprintf("release %s %s", project.Name, project.Version)); err != nil {
+					return fmt.Errorf("creating tag %s: %w", tag, err)
+				}
+				if err := git.PushTag(opts.RepoRoot, tag); err != nil {
+					return fmt.Errorf("pushing tag %s: %w", tag, err)
+				}
+				fmt.Printf("created and pushed tag %s\n", tag)
+			}
+		}
+	}
+
+	if !project.Publish.PublishReleases() {
+		return nil
+	}
+
+	if opts.RepoSlug == "" {
+		return fmt.Errorf("STAMP_REPO (owner/repo) must be set to publish releases")
+	}
+
+	// --- Collect artifact paths ---
+	var artifacts []string
+	for _, pattern := range project.Publish.Artifacts {
+		matches, err := filepath.Glob(filepath.Join(opts.RepoRoot, pattern))
+		if err != nil {
+			return fmt.Errorf("glob %q: %w", pattern, err)
+		}
+		artifacts = append(artifacts, matches...)
+	}
+
+	releaseTitle := fmt.Sprintf("%s v%s", project.Name, project.Version)
+	releaseBody := fmt.Sprintf("Release of %s version %s", project.Name, project.Version)
+
+	// Detect host: GitHub vs Gitea
+	if isGitea() {
+		if err := publishGitea(opts, tag, releaseTitle, releaseBody, artifacts); err != nil {
+			return err
+		}
+	} else {
+		if err := publishGitHub(opts, tag, releaseTitle, releaseBody, artifacts); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func publishGitHub(opts Options, tag, title, body string, artifacts []string) error {
+	client, err := ghClient.NewClient(opts.RepoSlug)
+	if err != nil {
+		return err
+	}
+
+	ctx := context.Background()
+
+	if opts.DryRun {
+		fmt.Printf("[dry-run] would create GitHub release %s with %d artifact(s)\n", tag, len(artifacts))
+		return nil
+	}
+
+	releaseID, err := client.CreateRelease(ctx, tag, title, body, false, false)
+	if err != nil {
+		return err
+	}
+	fmt.Printf("created GitHub release %s (id=%d)\n", tag, releaseID)
+
+	for _, a := range artifacts {
+		if err := client.UploadAsset(ctx, releaseID, a); err != nil {
+			return err
+		}
+		fmt.Printf("uploaded artifact: %s\n", a)
+	}
+	return nil
+}
+
+func publishGitea(opts Options, tag, title, body string, artifacts []string) error {
+	client, err := giteaClient.NewClient(opts.RepoSlug)
+	if err != nil {
+		return err
+	}
+
+	if opts.DryRun {
+		fmt.Printf("[dry-run] would create Gitea release %s with %d artifact(s)\n", tag, len(artifacts))
+		return nil
+	}
+
+	releaseID, err := client.CreateRelease(tag, title, body, false, false)
+	if err != nil {
+		return err
+	}
+	fmt.Printf("created Gitea release %s (id=%d)\n", tag, releaseID)
+
+	for _, a := range artifacts {
+		if err := client.UploadAsset(releaseID, a); err != nil {
+			return err
+		}
+		fmt.Printf("uploaded artifact: %s\n", a)
+	}
+	return nil
+}
+
+// isGitea returns true when Gitea environment variables are configured.
+func isGitea() bool {
+	return os.Getenv("GITEA_BASE_URL") != ""
+}