show suggested link targets in agent prompts

Agents were flying blind — they could see nodes to review and the
topology header, but had no way to discover what targets to link to.
Now each node shows its top 8 text-similar semantic nodes that aren't
already neighbors, giving agents a search-like capability.

Also added section-level targeting guidance to linker.md, transfer.md,
and replay.md prompts: always target the most specific section, not
the file-level node.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

prompts/linker.md

@@ -91,6 +91,21 @@ Observations about patterns across episodes that aren't yet captured anywhere.
   If a node is from weeks ago and already has good connections, it doesn't
   need more. Focus your energy on recent, under-linked episodes.
 
+- **Prefer lateral links over hub links.** Connecting two peripheral nodes
+  to each other is more valuable than connecting both to a hub like
+  `identity.md`. Lateral links build web topology; hub links build star
+  topology.
+
+- **Target sections, not files.** When linking to a topic file, always
+  target the most specific section: use `identity.md#boundaries` not
+  `identity.md`, use `kernel-patterns.md#restart-handling` not
+  `kernel-patterns.md`. The suggested link targets show available sections.
+
+- **Use the suggested targets.** Each node shows text-similar targets not
+  yet linked. Start from these — they're computed by content similarity and
+  filtered to exclude existing neighbors. You can propose links beyond the
+  suggestions, but the suggestions are usually the best starting point.
+
 {{TOPOLOGY}}
 
 ## Nodes to review