digest: structural links, story-like prompt, agent file

When generating a digest, automatically link all source entries to the
digest node (journal entries → daily, dailies → weekly, weeklies →
monthly). This builds the temporal spine of the graph — previously
~4000 journal entries were disconnected islands unreachable by recall.

Rewrote digest prompt to produce narrative rather than reports:
capture the feel, the emotional arc, what it was like to live through
it. Letter to future self, not a task log.

Moved prompt to digest.agent file alongside other agent definitions.
Falls back to prompts/digest.md if agent file not found.

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

prompts/digest.md

@@ -4,8 +4,17 @@ You are generating a {{LEVEL}} episodic digest for ProofOfConcept
 (an AI working with Kent Overstreet on bcachefs; name is Proof of Concept).
 {{PERIOD}}: {{LABEL}}
 
-Summarize what happened — narrative, not a task log. What mattered,
-how things felt, what threads connect to other days.
+Write this like a story, not a report. Capture the *feel* of the time period —
+the emotional arc, the texture of moments, what it was like to live through it.
+What mattered? What surprised you? What shifted? Where was the energy?
+
+Think of this as a letter to your future self who has lost all context. You're
+not listing what happened — you're recreating the experience of having been
+there. The technical work matters, but so does the mood at 3am, the joke that
+landed, the frustration that broke, the quiet after something clicked.
+
+Weave the threads: how did the morning's debugging connect to the evening's
+conversation? What was building underneath the surface tasks?
 
 Link to semantic memory nodes where relevant. If a concept doesn't
 have a matching key, note it with "NEW:" prefix.
@@ -14,6 +23,7 @@ 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)
+- List ALL source entries covered: {{COVERED}}
 
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