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/github/github.go

@@ -0,0 +1,96 @@
+package github
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"strings"
+
+	gh "github.com/google/go-github/v70/github"
+)
+
+// Client wraps the GitHub REST API.
+type Client struct {
+	client *gh.Client
+	owner  string
+	repo   string
+}
+
+// NewClient creates a GitHub client from the GITHUB_TOKEN environment variable.
+// The repoSlug must be in "owner/repo" format.
+func NewClient(repoSlug string) (*Client, error) {
+	token := os.Getenv("GITHUB_TOKEN")
+	if token == "" {
+		return nil, fmt.Errorf("GITHUB_TOKEN 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)
+	}
+	return &Client{
+		client: gh.NewClient(nil).WithAuthToken(token),
+		owner:  parts[0],
+		repo:   parts[1],
+	}, nil
+}
+
+// CreateRelease creates a GitHub release for the given tag.
+func (c *Client) CreateRelease(ctx context.Context, tag, name, body string, draft, prerelease bool) (int64, error) {
+	rel, _, err := c.client.Repositories.CreateRelease(ctx, c.owner, c.repo, &gh.RepositoryRelease{
+		TagName:    gh.Ptr(tag),
+		Name:       gh.Ptr(name),
+		Body:       gh.Ptr(body),
+		Draft:      gh.Ptr(draft),
+		Prerelease: gh.Ptr(prerelease),
+	})
+	if err != nil {
+		return 0, fmt.Errorf("creating GitHub release %s: %w", tag, err)
+	}
+	return rel.GetID(), nil
+}
+
+// UploadAsset uploads a file to an existing release.
+func (c *Client) UploadAsset(ctx context.Context, 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.Repositories.UploadReleaseAsset(ctx, c.owner, c.repo, releaseID,
+		&gh.UploadOptions{Name: fileName}, f)
+	if err != nil {
+		return fmt.Errorf("uploading %s to release %d: %w", fileName, releaseID, err)
+	}
+	return nil
+}
+
+const commentMarker = "<!-- stamp-pr-comment -->"
+
+// UpsertPRComment posts or updates a stamped PR comment (identified by a hidden marker).
+func (c *Client) UpsertPRComment(ctx context.Context, prNumber int, body string) error {
+	markedBody := commentMarker + "\n" + body
+
+	// List existing comments to find ours.
+	comments, _, err := c.client.Issues.ListComments(ctx, c.owner, c.repo, prNumber, nil)
+	if err != nil {
+		return fmt.Errorf("listing PR comments: %w", err)
+	}
+
+	for _, comment := range comments {
+		if strings.Contains(comment.GetBody(), commentMarker) {
+			// Update existing comment.
+			_, _, err = c.client.Issues.EditComment(ctx, c.owner, c.repo, comment.GetID(),
+				&gh.IssueComment{Body: gh.Ptr(markedBody)})
+			return err
+		}
+	}
+
+	// Create new comment.
+	_, _, err = c.client.Issues.CreateComment(ctx, c.owner, c.repo, prNumber,
+		&gh.IssueComment{Body: gh.Ptr(markedBody)})
+	return err
+}