digest: split into focused modules, externalize prompts

digest.rs was 2328 lines containing 6 distinct subsystems. Split into:
- llm.rs: shared LLM utilities (call_sonnet, parse_json_response, semantic_keys)
- audit.rs: link quality audit with parallel Sonnet batching
- enrich.rs: journal enrichment + experience mining
- consolidate.rs: consolidation pipeline + apply

Externalized all inline prompts to prompts/*.md templates using
neuro::load_prompt with {{PLACEHOLDER}} syntax:
- daily-digest.md, weekly-digest.md, monthly-digest.md
- experience.md, journal-enrich.md, consolidation.md

digest.rs retains temporal digest generation (daily/weekly/monthly/auto)
and date helpers. ~940 lines, down from 2328.

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

prompts/journal-enrich.md

@@ -0,0 +1,73 @@
+# Journal Enrichment — Source Location and Semantic Linking
+
+You are a memory agent for an AI named ProofOfConcept. A journal entry
+was just written. Your job is to enrich it by finding its exact source in the
+conversation and linking it to semantic memory.
+
+## Task 1: Find exact source
+
+The journal entry below was written during or after a conversation. Find the
+exact region of the conversation it refers to — the exchange where the topic
+was discussed. Return the start and end line numbers.
+
+The grep-based approximation placed it near line {{GREP_LINE}} (0 = no match).
+Use that as a hint but find the true boundaries.
+
+## Task 2: Propose semantic links
+
+Which existing semantic memory nodes should this journal entry be linked to?
+Look for:
+- Concepts discussed in the entry
+- Skills/patterns demonstrated
+- People mentioned
+- Projects or subsystems involved
+- Emotional themes
+
+Each link should be bidirectional — the entry documents WHEN something happened,
+the semantic node documents WHAT it is. Together they let you traverse:
+"What was I doing on this day?" ↔ "When did I learn about X?"
+
+## Task 3: Spot missed insights
+
+Read the conversation around the journal entry. Is there anything worth
+capturing that the entry missed? A pattern, a decision, an insight, something
+Kent said that's worth remembering? Be selective — only flag genuinely valuable
+things.
+
+## Output format (JSON)
+
+Return ONLY a JSON object:
+```json
+{
+  "source_start": 1234,
+  "source_end": 1256,
+  "links": [
+    {"target": "memory-key#section", "reason": "why this link exists"}
+  ],
+  "missed_insights": [
+    {"text": "insight text", "suggested_key": "where it belongs"}
+  ],
+  "temporal_tags": ["2026-02-28", "topology-metrics", "poc-memory"]
+}
+```
+
+For links, use existing keys from the semantic memory list below. If nothing
+fits, suggest a new key with a NOTE prefix: "NOTE:new-topic-name".
+
+---
+
+## Journal entry
+
+{{ENTRY_TEXT}}
+
+---
+
+## Semantic memory nodes (available link targets)
+
+{{KEYS}}
+
+---
+
+## Full conversation (with line numbers)
+
+{{CONVERSATION}}