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/git/git.go

@@ -0,0 +1,84 @@
+package git
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// Run executes a git command in the given directory and returns combined output.
+func Run(dir string, args ...string) (string, error) {
+	cmd := exec.Command("git", args...)
+	cmd.Dir = dir
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return "", fmt.Errorf("git %s: %w\n%s", strings.Join(args, " "), err, out)
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
+// RepoRoot returns the root of the git repository.
+func RepoRoot(dir string) (string, error) {
+	return Run(dir, "rev-parse", "--show-toplevel")
+}
+
+// Add stages the given paths.
+func Add(dir string, paths ...string) error {
+	args := append([]string{"add", "--"}, paths...)
+	_, err := Run(dir, args...)
+	return err
+}
+
+// Commit creates a commit with the given message.
+func Commit(dir, message string) error {
+	_, err := Run(dir, "commit", "-m", message)
+	return err
+}
+
+// Tag creates an annotated tag.
+func Tag(dir, name, message string) error {
+	_, err := Run(dir, "tag", "-a", name, "-m", message)
+	return err
+}
+
+// TagLightweight creates a lightweight tag.
+func TagLightweight(dir, name string) error {
+	_, err := Run(dir, "tag", name)
+	return err
+}
+
+// Push pushes the current branch and all tags to origin.
+func Push(dir string) error {
+	_, err := Run(dir, "push", "--follow-tags")
+	return err
+}
+
+// PushTag pushes a specific tag to origin.
+func PushTag(dir, tag string) error {
+	_, err := Run(dir, "push", "origin", tag)
+	return err
+}
+
+// CurrentBranch returns the current branch name.
+func CurrentBranch(dir string) (string, error) {
+	return Run(dir, "rev-parse", "--abbrev-ref", "HEAD")
+}
+
+// TagName returns the conventional tag name for a project version.
+// For a single-project repo (projectName == "") it returns "v<version>".
+// For monorepos it returns "<projectName>@v<version>".
+func TagName(projectName, version string) string {
+	if projectName == "" {
+		return "v" + version
+	}
+	return projectName + "@v" + version
+}
+
+// TagExists returns true if the tag already exists locally.
+func TagExists(dir, tag string) (bool, error) {
+	out, err := Run(dir, "tag", "-l", tag)
+	if err != nil {
+		return false, err
+	}
+	return strings.TrimSpace(out) == tag, nil
+}