swarmctl: add CLI reference docs, same pattern as hivectl

Adds swarmctl markdown-docs (a hidden Verb, same clap-markdown +
hide=true shape as hivectl markdown-docs) and generates
docs/tools/swarmctl-cli.md from it. Wires a swarmctl-docs freshness
check into nix/checks.nix, same shape as hivectl-docs, diffing against
packages.swarmctl.

One real gotcha: PathArgs::resolve() reads required
SWARMCTL_AUTHELIA_* deployment env vars and errors if unset -
swarmctl markdown-docs must not go through that path (it needs none of
those vars, and the docs build runs it outside any real deployment).
Restructured main() so resolve() only runs for the User arm, not
unconditionally before the match.

Also links the new doc from docs/tools/README.md (new 'for the swarm
operator' section), CLAUDE.md's swarmctl bullet, and
docs/conventions.md's flake-check list.

Verified: cargo check/clippy -D warnings/test/fmt -p swarmctl all
clean; swarmctl markdown-docs diffs clean against the committed doc
(checked against both a plain cargo build and the actual nix build.
#swarmctl output); scripts/check-issue-refs.sh clean.

docs/conventions.md

@@ -506,6 +506,11 @@ check derivations they don't:
   flake check catches the drift, and `ci-log` often can't show you
   why (it 500s on a fast failure), so you're left guessing "builder
   flake" when it's a stale doc.
+- **`swarmctl-docs`** is the same check for `swarmctl` /
+  `docs/tools/swarmctl-cli.md`:
+  ```sh
+  nix develop -c cargo run --bin swarmctl -- markdown-docs > docs/tools/swarmctl-cli.md
+  ```
 - there's also a flake `cargo-test` check and NixOS module
   evaluation in the set.