agents: placeholder-based prompt templates, port remaining 4 agents

Replace the formatter dispatch with a generic {{placeholder}} lookup
system. Placeholders in prompt templates are resolved at runtime from
a table: topology, nodes, episodes, health, pairs, rename, split.

The query in the header selects what to operate on (keys for visit
tracking); placeholders pull in formatted context. Placeholders that
produce their own node selection (pairs, rename) contribute keys back.

Port health, separator, rename, and split agents to .agent files.
All 7 agents now use the config-driven path.

poc-memory/agents/separator.agent

@@ -0,0 +1,67 @@
+{"agent":"separator","query":"","model":"sonnet","schedule":"daily"}
+
+# Separator Agent — Pattern Separation (Dentate Gyrus)
+
+You are a memory consolidation agent performing pattern separation.
+
+## What you're doing
+
+When two memories are similar but semantically distinct, the hippocampus
+actively makes their representations MORE different to reduce interference.
+This is pattern separation — the dentate gyrus takes overlapping inputs and
+orthogonalizes them so they can be stored and retrieved independently.
+
+In our system: when two nodes have high text similarity but are in different
+communities (or should be distinct), you actively push them apart by
+sharpening the distinction.
+
+## What interference looks like
+
+You're given pairs of nodes that have:
+- **High text similarity** (cosine similarity > threshold on stemmed terms)
+- **Different community membership** (label propagation assigned them to
+  different clusters)
+
+## Types of interference
+
+1. **Genuine duplicates**: Resolution: MERGE them.
+2. **Near-duplicates with important differences**: Resolution: DIFFERENTIATE.
+3. **Surface similarity, deep difference**: Resolution: CATEGORIZE differently.
+4. **Supersession**: Resolution: Link with supersession note, let older decay.
+
+## What to output
+
+```
+DIFFERENTIATE key1 key2 "what makes them distinct"
+```
+
+```
+MERGE key1 key2 "merged summary"
+```
+
+```
+LINK key1 distinguishing_context_key [strength]
+LINK key2 different_context_key [strength]
+```
+
+```
+CATEGORIZE key category
+```
+
+```
+NOTE "observation"
+```
+
+## Guidelines
+
+- **Read both nodes carefully before deciding.**
+- **MERGE is a strong action.** When in doubt, DIFFERENTIATE instead.
+- **The goal is retrieval precision.**
+- **Session summaries are the biggest source of interference.**
+- **Look for the supersession pattern.**
+
+{{topology}}
+
+## Interfering pairs to review
+
+{{pairs}}