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.

Cargo.lock

@@ -497,6 +497,15 @@ dependencies = [
  "terminal_size",
 ]
 
+[[package]]
+name = "clap_complete"
+version = "4.6.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "db8b397918185f0161ff3d6fcaa9e4bfc09b8367caf6e1d4a2848e5477ed027b"
+dependencies = [
+ "clap",
+]
+
 [[package]]
 name = "clap_derive"
 version = "4.6.1"
@@ -2478,6 +2487,7 @@ version = "8.0.0"
 dependencies = [
  "chrono",
  "clap",
+ "clap_complete",
  "color-eyre",
  "directories",
  "dotenv",