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>

go.mod

@@ -0,0 +1,22 @@
+module github.com/thokra/stamp
+
+go 1.26.1
+
+require (
+	code.gitea.io/sdk/gitea v0.23.2
+	github.com/BurntSushi/toml v1.6.0
+	github.com/Masterminds/semver/v3 v3.4.0
+	github.com/google/go-github/v70 v70.0.0
+	github.com/urfave/cli/v3 v3.7.0
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require (
+	github.com/42wim/httpsig v1.2.3 // indirect
+	github.com/davidmz/go-pageant v1.0.2 // indirect
+	github.com/go-fed/httpsig v1.1.0 // indirect
+	github.com/google/go-querystring v1.1.0 // indirect
+	github.com/hashicorp/go-version v1.7.0 // indirect
+	golang.org/x/crypto v0.39.0 // indirect
+	golang.org/x/sys v0.33.0 // indirect
+)