Add TUI add and preview commands

Use charmbracelet/huh for an interactive add flow. Add a preview
subcommand to enter/exit per-project pre-release tags. Move stamp.toml
into .stamp/ and add ConfigPath helper. Prefer --snapshot then project
PreTag when computing versions and promote bumps to pre-release when
appropriate. Export CurrentVersion and add required TUI deps to go.mod.

examples/stamp.toml

@@ -1,14 +1,10 @@
-# stamp.toml — annotated example configuration
+# .stamp/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).