docs+comments: say what changed instead of tagging the tracker item

The prose added by this branch named the tracker item in seventeen
places, which check-issue-refs.sh rejects: a `#N` tag is dead weight for
anyone reading the public mirror, where no issue data exists. Each one
now states the fact it was pointing at — the parent field is gone — so
the sentence stands on its own.

Two of those lines also carried a rustdoc break: `[`write`]` in
topology.rs is ambiguous between the module's own `write` fn and the
`write!` macro, which `-D rustdoc::broken-intra-doc-links` fails. Spelled
`[`write()`]`, per rustdoc's own suggestion.

The host_config.rs rewrite is two lines rather than three so the doc
block stays under check-comment-blocks.sh's 30-line ceiling.

hive-agent-mcp/src/send_allow.rs

@@ -25,7 +25,7 @@ pub fn check_send_allowed(to: &str) -> Result<(), String> {
         //
         // This bypass used to be spelled `<parent>`, which the broker
         // resolved per `topology.json` and which fell back to `operator`
-        // for a root agent. #4472 removed the parent field, so every
+        // for a root agent. The parent field is gone now, so every
         // agent is what that fallback called a root — the exemption is
         // now written as the name it always resolved to. Same reachable
         // set, one fewer indirection.