feat(3216): swarmctl shell completions

Mirrors hivectl exactly: a `completions <shell>` verb that walks the
live clap tree, and a package that pipes it into installShellCompletion
for bash/zsh/fish. Generating from the command tree rather than writing a
script by hand is what keeps completions from drifting away from the
verbs they complete — the same reason `markdown-docs` renders the docs
from that tree.

Dispatched before PathArgs::resolve() for the same reason markdown-docs
is: emitting a completion script needs none of the SWARMCTL_AUTHELIA_*
deployment env vars, and requiring them would make the package's own
build-time invocation fail — exactly where it runs.

swarmctl leaves mkBinPackage for its own derivation, since the extractor
installs a binary and nothing else.

docs/tools/swarmctl-cli.md

@@ -8,6 +8,7 @@ This document contains the help content for the `swarmctl` command-line program.
 * [`swarmctl user`↴](#swarmctl-user)
 * [`swarmctl user add`↴](#swarmctl-user-add)
 * [`swarmctl user update`↴](#swarmctl-user-update)
+* [`swarmctl completions`↴](#swarmctl-completions)
 
 ## `swarmctl`
 
@@ -18,6 +19,7 @@ swarm-level operator CLI
 ###### **Subcommands:**
 
 * `user` — Manage subjects in the swarm's SSO provider
+* `completions` — Generate a shell completion script for `swarmctl` and print it to stdout
 
 ###### **Options:**
 
@@ -81,6 +83,25 @@ Every flag is optional and they compose, so one call can set several things at o
 
 
 
+## `swarmctl completions`
+
+Generate a shell completion script for `swarmctl` and print it to stdout.
+
+Supports bash, zsh, fish, elvish and powershell. The nix package already installs bash/zsh/fish system-wide; this is for ad-hoc or other-shell use.
+
+Dispatched before `PathArgs::resolve()` for the same reason as `markdown-docs`: emitting a completion script needs none of the `SWARMCTL_AUTHELIA_*` deployment env vars, and requiring them would make the package's own build-time invocation fail.
+
+**Usage:** `swarmctl completions <SHELL>`
+
+###### **Arguments:**
+
+* `<SHELL>` — Shell to emit completions for
+
+  Possible values: `bash`, `elvish`, `fish`, `powershell`, `zsh`
+
+
+
+
 <hr/>
 
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