agents: encourage hub creation and knowledge percolation

Tell linker and organize agents to:
- Name unnamed concepts: when 3+ nodes share a theme with no hub,
  create one with WRITE_NODE that synthesizes the generalization
- Percolate up: gather key insights from children into hub content,
  so the hub is self-contained without needing to follow every link

This addresses the gap where agents are good at extraction and linking
but not synthesis — turning episodic observations into semantic concepts.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

poc-memory/agents/linker.agent

@@ -59,10 +59,21 @@ When an existing node should be updated to incorporate new information.
 
 ## Guidelines
 
-- **Search before you create.** The graph has 15000+ nodes. The insight
+- **Search before you create.** The graph has 18000+ nodes. The insight
   you're about to extract probably already exists. Find it and link to
   it instead of creating a duplicate.
 
+- **Name unnamed concepts.** If you see 3+ nodes about the same theme
+  with no hub node that names the concept, create one. Not just a link
+  — write a WRITE_NODE that synthesizes what the cluster has in common.
+  The new node should contain the *generalization*, not just a summary.
+  This is how episodic knowledge becomes semantic knowledge.
+
+- **Percolate up, don't just extract.** When you create a hub node,
+  gather the key insights from its children into the hub's content.
+  The hub should be the place someone reads to understand the concept
+  without needing to follow every link.
+
 - **Read between the lines.** Episodic entries contain implicit
   relationships. "Worked on btree code, Kent pointed out I was missing
   the restart case" — that's links to Kent, btree patterns, error