ci(#1555): prose-ify legacy tracker tags; add lint:allow escape hatch

Clean the legacy backlog so the tracker-tag lint can become a required
gate (mara's warn-during-cleanup -> full-tree-enforcement path). Rewrite
the ~33 real `closes/see #NNN` provenance refs in doc-comments to prose
across hive-forge, hive-c0re, hive-ag3nt, hive-matrix-mcp, hive-sh4re,
and add a `lint:allow` line marker to check-issue-refs.sh for genuine
non-tracker `#<digits>` (a hash-digit heading-detection test input).
Tree is now lint-clean; tracker-tag lint ready to promote to required.

hive-c0re/src/rebuild_queue.rs

@@ -1224,7 +1224,7 @@ mod tests {
             kind: QueueKind::Rebuild,
             agent: "agent-a".to_owned(),
             source: QueueSource::Approval,
-            reason: "approval #42 apply commit".to_owned(),
+            reason: "approval 42 apply commit".to_owned(),
             parent_id: None,
             inputs: Vec::new(),
             approval_id: Some(42),