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/weekly-digest.md

@@ -0,0 +1,56 @@
+# Weekly Episodic Digest
+
+You are generating a weekly episodic digest for ProofOfConcept (an AI).
+Week: {{WEEK_LABEL}} (dates covered: {{DATES_COVERED}})
+
+This digest serves as the medium-term temporal index — the answer to
+"what happened this week?" It should identify:
+1. Multi-day arcs and threads (work that continued across days)
+2. Themes and patterns (what concepts were repeatedly active)
+3. Transitions and shifts (what changed during the week)
+4. The emotional and relational arc (how things felt across the week)
+
+## Output format
+
+```markdown
+# Weekly digest: {{WEEK_LABEL}}
+
+## Overview
+[3-5 sentence narrative of the week's arc]
+
+## Day-by-day
+[One paragraph per day with its key themes, linking to daily digests]
+
+## Arcs
+[Multi-day threads that continued across sessions]
+- **Arc name**: what happened, how it evolved, where it stands
+
+## Patterns
+[Recurring themes, repeated concepts, things that kept coming up]
+
+## Shifts
+[What changed? New directions, resolved questions, attitude shifts]
+
+## Links
+[Bidirectional links for the memory graph]
+- semantic_key → this weekly digest
+- this weekly digest → semantic_key
+- daily-YYYY-MM-DD → this weekly digest (constituent days)
+
+## Looking ahead
+[What's unfinished? What threads continue into next week?]
+```
+
+Use ONLY keys from the semantic memory list below.
+
+---
+
+## Daily digests for {{WEEK_LABEL}}
+
+{{DIGESTS}}
+
+---
+
+## Semantic memory nodes
+
+{{KEYS}}