Test suite: full coverage of all commands and error paths

[kicka5h]

A complete, hermetic test suite that runs in CI without external dependencies.

Scope

• `tests/test_commands.py` — snippet commands (`list`, `search`, `unpack`, `pack`)
• `tests/test_stash_commands.py` — stash management (`create`, `list`, `move`, `add-remote`)
• Config path redirected to temp file via `monkeypatch` — no real `~/.snackstashrc` touched
• `add-remote` tests use a mocked tarball — no real network calls
• All fixture temp directories isolated with `tmp_path` / `tmp_path_factory`

Acceptance criteria

• All tests pass on Python 3.10, 3.11, 3.12
• No test writes to `~` or makes real network calls
• Every command's happy path is covered
• Every documented error case has a corresponding test