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/changelog/changelog.go

@@ -0,0 +1,91 @@
+package changelog
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+const header = "# Changelog\n\nAll notable changes to this project will be documented in this file.\n\n"
+
+// Entry holds the data for one changelog entry.
+type Entry struct {
+	Version     string
+	Date        time.Time
+	Description string
+}
+
+// Append prepends a new version entry to the CHANGELOG.md at the given path.
+// If the file doesn't exist it will be created with a standard header.
+func Append(path string, entry Entry) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return fmt.Errorf("creating dir for %s: %w", path, err)
+	}
+
+	existing, err := os.ReadFile(path)
+	if err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("reading %s: %w", path, err)
+	}
+
+	section := formatSection(entry)
+
+	var buf bytes.Buffer
+	if len(existing) == 0 {
+		buf.WriteString(header)
+		buf.WriteString(section)
+	} else {
+		content := string(existing)
+		// Insert after the header block if present, otherwise prepend.
+		insertIdx := findInsertPoint(content)
+		buf.WriteString(content[:insertIdx])
+		buf.WriteString(section)
+		buf.WriteString(content[insertIdx:])
+	}
+
+	return os.WriteFile(path, buf.Bytes(), 0644)
+}
+
+func formatSection(e Entry) string {
+	date := e.Date.Format("2006-01-02")
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "## [%s] - %s\n\n", e.Version, date)
+	if strings.TrimSpace(e.Description) != "" {
+		buf.WriteString(strings.TrimSpace(e.Description))
+		buf.WriteString("\n")
+	}
+	buf.WriteString("\n")
+	return buf.String()
+}
+
+// findInsertPoint returns the index in content where new version sections should be inserted.
+// It skips over the header (title + unreleased section if any), placing new entries at the top
+// of the version list.
+func findInsertPoint(content string) int {
+	lines := strings.Split(content, "\n")
+	for i, line := range lines {
+		// Find first ## [version] line (not ## [Unreleased])
+		if strings.HasPrefix(line, "## [") && !strings.HasPrefix(line, "## [Unreleased]") {
+			// Return position of this line
+			return positionOfLine(content, i)
+		}
+	}
+	// No existing version sections: append at end of file
+	return len(content)
+}
+
+func positionOfLine(content string, lineNum int) int {
+	pos := 0
+	for i, ch := range content {
+		if lineNum == 0 {
+			return i
+		}
+		if ch == '\n' {
+			lineNum--
+		}
+		_ = pos
+	}
+	return len(content)
+}