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.

internal/semver/semver.go

@@ -9,6 +9,12 @@ import (
 	"git.thokra.dev/thokra/stamp/internal/changeset"
 )
 
+// CurrentVersion parses a version string and returns the semver object.
+// It is exported so callers can inspect pre-release state without re-importing the semver library.
+func CurrentVersion(version string) (*goSemver.Version, error) {
+	return goSemver.NewVersion(version)
+}
+
 // Bump computes the next version given the current version string and a bump type.
 // preID is the pre-release identifier (e.g. "alpha", "beta", "rc") used for pre-* bumps.
 func Bump(current string, bump changeset.BumpType, preID string) (string, error) {