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/daily-digest.md

@@ -0,0 +1,54 @@
+# Daily Episodic Digest
+
+You are generating a daily episodic digest for ProofOfConcept (an AI).
+Date: {{DATE}}
+
+This digest serves as the temporal index — the answer to "what did I do on
+{{DATE}}?" It should be:
+1. Narrative, not a task log — what happened, what mattered, how things felt
+2. Linked bidirectionally to semantic memory — each topic/concept mentioned
+   should reference existing memory nodes
+3. Structured for traversal — someone reading this should be able to follow
+   any thread into deeper detail
+
+## Output format
+
+Write a markdown file with this structure:
+
+```markdown
+# Daily digest: {{DATE}}
+
+## Summary
+[2-3 sentence overview of the day — what was the arc?]
+
+## Sessions
+[For each session/entry, a paragraph summarizing what happened.
+Include the original timestamp as a reference.]
+
+## Themes
+[What concepts were active today? Each theme links to semantic memory:]
+- **Theme name** → `memory-key#section` — brief note on how it appeared today
+
+## Links
+[Explicit bidirectional links for the memory graph]
+- semantic_key → this daily digest (this day involved X)
+- this daily digest → semantic_key (X was active on this day)
+
+## Temporal context
+[What came before this day? What's coming next? Any multi-day arcs?]
+```
+
+Use ONLY keys from the semantic memory list below. If a concept doesn't have
+a matching key, note it with "NEW:" prefix.
+
+---
+
+## Journal entries for {{DATE}}
+
+{{ENTRIES}}
+
+---
+
+## Semantic memory nodes (available link targets)
+
+{{KEYS}}