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

docs/swarm/README.md

@@ -425,7 +425,7 @@ Headers go out **on transition, not on a timer**: the harness rebuilds the
 header whenever its event bus moves and publishes only when the result differs
 from what it last sent. That's the whole point of the subject — the
 `agent-status` bucket already republishes once a minute, which is far too slow
-for "is this agent thinking right now". The cost of a core subject is that a
+for "is this agent thinking right now." The cost of a core subject is that a
 subscriber attaching mid-idle sees nothing until the next change, so a renderer
 opens with the bucket's snapshot and lets this stream refine it.
 

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.