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.

hive-c0re/src/bin/hivectl.rs

@@ -157,6 +157,18 @@ enum Cmd {
     /// hand-maintained command list to drift out of date.
     #[command(hide = true)]
     MarkdownDocs,
+    /// Generate a shell completion script for `hivectl` and print it to
+    /// stdout.
+    ///
+    /// Pipe it into your shell's completion path — e.g. for zsh:
+    /// `hivectl completions zsh > ~/.zsh/completions/_hivectl` (with that
+    /// dir on `$fpath`). The hyperhive NixOS module installs the zsh script
+    /// system-wide automatically, so this is mainly for ad-hoc / other-shell
+    /// use. Supports bash, zsh, fish, elvish, and powershell.
+    Completions {
+        /// Shell to emit completions for.
+        shell: clap_complete::Shell,
+    },
 }
 
 /// Shared scope flags for `hivectl stop` / `hivectl start`. With no flag
@@ -441,9 +453,22 @@ async fn main() -> Result<()> {
             print!("{}", clap_markdown::help_markdown::<Cli>());
             Ok(())
         }
+        Cmd::Completions { shell } => {
+            generate_completions(shell);
+            Ok(())
+        }
     }
 }
 
+/// Emit a shell completion script for `hivectl` to stdout. Walks the clap
+/// command tree (the single source of truth — same tree `markdown-docs`
+/// renders) so completions never drift from the actual verbs/flags.
+fn generate_completions(shell: clap_complete::Shell) {
+    use clap::CommandFactory as _;
+    let mut cmd = Cli::command();
+    clap_complete::generate(shell, &mut cmd, "hivectl", &mut std::io::stdout());
+}
+
 /// True when `name` matches an existing hyperhive agent — i.e. it has a
 /// persistent state dir under `/var/lib/hyperhive/agents/`. We use the
 /// state dir (not the live container list) so kept-state tombstones