digest: unify generators and prompts across all three levels

Three near-identical generate_daily/weekly/monthly functions collapsed
into one generate_digest() parameterized by DigestLevel descriptors.
Three separate prompt templates merged into one prompts/digest.md with
level-specific instructions carried in the DigestLevel struct.

Each level defines: name, title, period label, input title, output
format instructions, child prefix (None for daily = reads journal),
and Sonnet timeout.

digest_auto simplified correspondingly — same three phases but using
the unified generator.

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

prompts/digest.md

@@ -0,0 +1,20 @@
+# {{LEVEL}} Episodic Digest
+
+You are generating a {{LEVEL}} episodic digest for ProofOfConcept (an AI).
+{{PERIOD}}: {{LABEL}}
+
+{{INSTRUCTIONS}}
+
+Use ONLY keys from the semantic memory list below.
+
+---
+
+## {{INPUT_TITLE}} for {{LABEL}}
+
+{{CONTENT}}
+
+---
+
+## Semantic memory nodes
+
+{{KEYS}}