feat: install tab completion for bash, zsh, and fish (#16)

`completions install` generates the script from the same clap definition
the binary parses with, so it cannot drift from `--help`, and writes it
where each shell already looks. fish scans its own completions directory
unprompted; bash and zsh get one marked `source` line appended to their rc
file, because nothing puts a per-user directory on their search path by
default. `completions uninstall` takes back exactly those lines and leaves
the rest of the file alone.

soulseek-rs/src/SKILL.md

@@ -97,6 +97,10 @@ answered, which is a negative answer rather than an error.
 this skill file into the local agent directories, or remove it: `agent`,
 `path`, `action`.
 
+**`completions install`** and **`completions uninstall`** — write the tab
+completion script where bash, zsh, or fish looks for it, or take it back
+again: `shell`, `path`, `action`. For a human at a prompt, not for you.
+
 ## Idioms
 
 ```bash