Stash management: create, list, move

[kicka5h]

Named stash management commands for registering, inspecting, and relocating stash directories.

Scope

• `snack stash create ` — register a new named stash, create directory, auto-activate if first
• `snack stash list` — display all stashes with active marker and missing-path warning
• `snack stash move ` — move stash on disk and update config
• Config file written in INI format to `~/.snackstashrc`
• Legacy `stash=<path` format still read for backwards compatibility

Acceptance criteria

• `create` writes correct INI config and creates the directory
• First stash is auto-activated; subsequent stashes respect `--activate`/`--no-activate`
• `list` flags stashes whose paths no longer exist on disk
• `move` relocates directory contents and updates config atomically
• Duplicate name, missing name, and occupied target path all produce clear errors
• Tests cover all commands and error paths