Snippet commands: list, search, unpack, pack

[kicka5h]

Core snippet workflow commands that form the primary interface of the tool.

Scope

• `snack list [category]` — list all `.py` files in the active stash, with optional category filter
• `snack search ` — case-insensitive filename search
• `snack unpack [--flat] [--force]` — copy from stash into cwd
• `snack pack [--force]` — copy from cwd into stash
• Overwrite prompts on conflict; `--force` to skip
• Clear error messages for missing files

Acceptance criteria

• All four commands work end-to-end with a real stash
• `--flat` drops the file into cwd with no subdirectory
• Conflict prompt is shown when destination exists; `--force` suppresses it
• Errors for missing source/destination are human-readable
• Tests cover happy path and all error cases