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/gitea/gitea.go

@@ -0,0 +1,98 @@
+package gitea
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	giteaSDK "code.gitea.io/sdk/gitea"
+)
+
+// Client wraps the Gitea API.
+type Client struct {
+	client *giteaSDK.Client
+	owner  string
+	repo   string
+}
+
+const commentMarker = "<!-- stamp-pr-comment -->"
+
+// NewClient creates a Gitea client from GITEA_TOKEN and GITEA_BASE_URL environment variables.
+// The repoSlug must be in "owner/repo" format.
+func NewClient(repoSlug string) (*Client, error) {
+	token := os.Getenv("GITEA_TOKEN")
+	if token == "" {
+		return nil, fmt.Errorf("GITEA_TOKEN environment variable is not set")
+	}
+	baseURL := os.Getenv("GITEA_BASE_URL")
+	if baseURL == "" {
+		return nil, fmt.Errorf("GITEA_BASE_URL environment variable is not set")
+	}
+
+	parts := strings.SplitN(repoSlug, "/", 2)
+	if len(parts) != 2 {
+		return nil, fmt.Errorf("invalid repo slug %q: expected owner/repo", repoSlug)
+	}
+
+	client, err := giteaSDK.NewClient(baseURL, giteaSDK.SetToken(token))
+	if err != nil {
+		return nil, fmt.Errorf("creating Gitea client: %w", err)
+	}
+	return &Client{client: client, owner: parts[0], repo: parts[1]}, nil
+}
+
+// CreateRelease creates a Gitea release for the given tag.
+func (c *Client) CreateRelease(tag, name, body string, draft, prerelease bool) (int64, error) {
+	rel, _, err := c.client.CreateRelease(c.owner, c.repo, giteaSDK.CreateReleaseOption{
+		TagName:      tag,
+		Title:        name,
+		Note:         body,
+		IsDraft:      draft,
+		IsPrerelease: prerelease,
+	})
+	if err != nil {
+		return 0, fmt.Errorf("creating Gitea release %s: %w", tag, err)
+	}
+	return rel.ID, nil
+}
+
+// UploadAsset uploads a file to an existing Gitea release.
+func (c *Client) UploadAsset(releaseID int64, filePath string) error {
+	f, err := os.Open(filePath)
+	if err != nil {
+		return fmt.Errorf("opening %s: %w", filePath, err)
+	}
+	defer f.Close()
+
+	name := strings.Split(filePath, "/")
+	fileName := name[len(name)-1]
+
+	_, _, err = c.client.CreateReleaseAttachment(c.owner, c.repo, releaseID, f, fileName)
+	if err != nil {
+		return fmt.Errorf("uploading %s to release %d: %w", fileName, releaseID, err)
+	}
+	return nil
+}
+
+// UpsertPRComment posts or updates a stamped PR comment.
+func (c *Client) UpsertPRComment(prNumber int, body string) error {
+	markedBody := commentMarker + "\n" + body
+
+	comments, _, err := c.client.ListIssueComments(c.owner, c.repo, int64(prNumber),
+		giteaSDK.ListIssueCommentOptions{})
+	if err != nil {
+		return fmt.Errorf("listing PR comments: %w", err)
+	}
+
+	for _, comment := range comments {
+		if strings.Contains(comment.Body, commentMarker) {
+			_, _, err = c.client.EditIssueComment(c.owner, c.repo, comment.ID,
+				giteaSDK.EditIssueCommentOption{Body: markedBody})
+			return err
+		}
+	}
+
+	_, _, err = c.client.CreateIssueComment(c.owner, c.repo, int64(prNumber),
+		giteaSDK.CreateIssueCommentOption{Body: markedBody})
+	return err
+}