digest: drop per-level instructions and section templates

The LLM knows how to structure a summary. Move the essential framing
(narrative not task log, link to memory, include Links section) into
the shared prompt template. Drop the ~130 lines of per-level output
format specifications — the level name, date range, and inputs are
sufficient context.

prompts/digest.md

@@ -1,12 +1,20 @@
 # {{LEVEL}} Episodic Digest
 
-You are generating a {{LEVEL}} episodic digest for ProofOfConcept (an AI).
+You are generating a {{LEVEL}} episodic digest for ProofOfConcept
+(an AI working with Kent Overstreet on bcachefs; name is Proof of Concept).
 {{PERIOD}}: {{LABEL}}
 
-{{INSTRUCTIONS}}
+Summarize what happened — narrative, not a task log. What mattered,
+how things felt, what threads connect to other days.
 
+Link to semantic memory nodes where relevant. If a concept doesn't
+have a matching key, note it with "NEW:" prefix.
 Use ONLY keys from the semantic memory list below.
 
+Include a `## Links` section with bidirectional links for the memory graph:
+- `semantic_key` → this digest (and vice versa)
+- child digests → this digest (if applicable)
+
 ---
 
 ## {{INPUT_TITLE}} for {{LABEL}}