docs/setup: use the emphasis form treefmt's prettier normalises to

The three cross-references this branch rewrote used `*Swarm SSO*`; prettier
emits `_Swarm SSO_`, so checks.formatting went red on an otherwise
content-only change.

Verified: nix fmt is now idempotent on this tree (0 changed); the three
scripts/check-*.sh lints exit 0 with the change staged.

docs/getting-started/setup.md

@@ -32,7 +32,7 @@ hivectl forge create-user ruth
 ```
 
 Swarm SSO creates the human operator's own forge account instead of
-a manual `hivectl` step — see *Swarm SSO* below (`swarmctl user add`).
+a manual `hivectl` step — see _Swarm SSO_ below (`swarmctl user add`).
 
 ### 2 · Gateway (HTTP Basic auth)
 
@@ -214,7 +214,7 @@ Nothing to run — it's served on the swarm apex
 (`https://<swarm.domain>/`) as soon as the host rebuilds. Two things
 decide whether you can actually open it:
 
-- **You are in `admins`** (*Swarm SSO* above). The gateway asks authelia whether
+- **You are in `admins`** (_Swarm SSO_ above). The gateway asks authelia whether
   you have a session; the rule that makes it mean _operator_ wants the
   group. Without it you log in and still get bounced.
 - **The name resolves to this host.** it's published to the hive's own
@@ -243,7 +243,7 @@ hivectl matrix promote-user mara
 ```
 
 Swarm SSO creates the human operator's own matrix account instead of
-a manual `hivectl` step — see *Swarm SSO* above (`swarmctl user add`).
+a manual `hivectl` step — see _Swarm SSO_ above (`swarmctl user add`).
 
 ### 7 · Spawn sub-agents