hivectl: shell completions verb + ship zsh/bash/fish completions (#1764)

Add a 'hivectl completions <shell>' subcommand (clap_complete) that
prints a completion script for bash/zsh/fish/elvish/powershell, generated
from hivectl's own clap command tree so it never drifts from the real
verbs/flags. The package build installs the bash/zsh/fish scripts via
installShellFiles, so an operator gets working completion automatically
once hivectl is on their profile with shell completion enabled.

Regenerated docs/tools/hivectl-cli.md for the new verb.

Cargo.lock

@@ -539,6 +539,15 @@ dependencies = [
  "strsim",
 ]
 
+[[package]]
+name = "clap_complete"
+version = "4.6.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e0a7a9bfdb35811f9e59832f0f05975114d2251b415fb534108e6f34060fd772"
+dependencies = [
+ "clap",
+]
+
 [[package]]
 name = "clap_derive"
 version = "4.6.1"
@@ -1354,6 +1363,7 @@ dependencies = [
  "bcrypt",
  "clap",
  "clap-markdown",
+ "clap_complete",
  "hive-sh4re",
  "libc",
  "listenfd",