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>

examples/stamp.toml

@@ -0,0 +1,46 @@
+# stamp.toml — annotated example configuration
+
+# Global settings (all optional)
+[config]
+# The base branch used by `stamp comment` to detect what's changed in a PR.
+base_branch = "main"
+
+# Directory where stamp files (.md changesets) are stored.
+# Default: .stamp
+changeset_dir = ".stamp"
+
+# --- Projects ---
+# Define one [[projects]] block per versioned component.
+# For single-project repos, define just one block with name = "" (or any name).
+
+[[projects]]
+name    = "my-app"
+path    = "apps/my-app"      # path relative to repo root
+version = "1.2.3"            # current version — updated by `stamp version`
+changelog = "CHANGELOG.md"   # relative to path (default: CHANGELOG.md)
+
+  [projects.publish]
+  # Create and push a git tag for this project on `stamp publish`.
+  # Tag format: <name>@v<version>  e.g. my-app@v1.2.3
+  tags = true
+
+  # Create a GitHub/Gitea release with the changelog section as the release body.
+  releases = true
+
+  # Glob patterns (relative to repo root) for files to upload as release artifacts.
+  artifacts = [
+    "apps/my-app/dist/my-app-linux-amd64",
+    "apps/my-app/dist/my-app-darwin-arm64",
+    "apps/my-app/dist/my-app-windows-amd64.exe",
+  ]
+
+[[projects]]
+name    = "my-lib"
+path    = "libs/my-lib"
+version = "0.1.0"
+
+  [projects.publish]
+  tags     = true
+  releases = true
+  # No artifacts for a library.
+  artifacts = []