fix: resolve unresolved rustdoc link and quote-punctuation lint

- swarm_agent_state.rs: HEADROOM in swarm_term.rs is private, so the
  intra-doc link can never resolve; switch to plain backticks.
- docs/swarm/README.md: move closing period inside the quoted phrase
  per Microsoft.Quotes.

Refs #3802

hive-agent/src/swarm_agent_state.rs

@@ -57,7 +57,7 @@ const CLIENT_ID_SUFFIX: &str = "-agent";
 
 /// How much room to leave under the announced limit for everything the
 /// publish adds around the payload — subject, headers, protocol framing.
-/// Same margin and same reasoning as [`crate::swarm_term::HEADROOM`].
+/// Same margin and same reasoning as `swarm_term::HEADROOM`.
 const HEADROOM: usize = 1024;
 
 /// One turn-state header, as a swarm-level reader receives it.