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/src/main.rs

@@ -282,7 +282,7 @@ trait Surface {
     /// of a container: this is where a turn failure or a plugin-install
     /// failure surfaces when nothing inside the harness can act on it.
     ///
-    /// Was `<parent>` before #4472, a sentinel the broker resolved per
+    /// Was `<parent>` while the hierarchy existed, a sentinel the broker resolved per
     /// `topology.json` and which already fell through to `operator` for a
     /// root agent. With the parent field gone every agent takes that
     /// branch, so the recipient is written out rather than resolved.