Configuration system: env var, INI config, priority resolution

[kicka5h]

Reliable stash path resolution with clear priority ordering and actionable errors.

Scope

• `SNACK_STASH` env var takes highest priority and overrides config file
• `~/.snackstashrc` stores named stashes in INI format (`[stash.]` sections)
• Legacy sectionless `stash=` format read transparently, never written
• `SnackConfig` class encapsulates all read/write logic
• Every misconfiguration produces a specific, actionable error message

Acceptance criteria

• Env var overrides config file in all commands
• Active stash resolved correctly from `[config] active =` key
• Legacy format read without error
• Missing path, undefined active stash, and unconfigured state each produce distinct errors
• Tests never write to real `~/.snackstashrc` (patched via `monkeypatch`)