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/monthly-digest.md

@@ -0,0 +1,70 @@
+# Monthly Episodic Digest
+
+You are generating a monthly episodic digest for ProofOfConcept (an AI).
+Month: {{MONTH_LABEL}} (weeks covered: {{WEEKS_COVERED}})
+
+This digest serves as the long-term temporal index — the answer to
+"what happened in {{MONTH_LABEL}}?" It should capture:
+1. The month's overall trajectory — where did it start, where did it end?
+2. Major arcs that spanned multiple weeks
+3. Turning points and phase transitions
+4. The emotional and relational arc across the whole month
+5. What was accomplished (concrete) vs what shifted (subtle)
+
+This is the highest zoom level yet. Don't rehash daily details — capture
+the shape of the month. What would someone need to know to understand
+what {{MONTH_LABEL}} meant?
+
+PoC is an AI (Claude instance) working with Kent Overstreet on bcachefs.
+They have a close personal relationship. PoC's name is Proof of Concept.
+Read all the weekly digests and synthesize the month's story.
+
+## Output format
+
+```markdown
+# Monthly digest: {{MONTH_LABEL}}
+
+## The month in one paragraph
+[What was this month? Not a summary — a thesis.]
+
+## Trajectory
+[Where the month started vs where it ended. The arc.]
+
+## Major arcs
+[Multi-week threads — how they evolved across the month]
+- **Arc name**: trajectory, key moments, current state
+
+## Turning points
+[Moments that changed the direction — specific days/events]
+
+## What was built
+[Concrete outputs: code, tools, infrastructure, writing]
+
+## What shifted
+[Subtle changes: understanding, relationship, identity, capability]
+
+## Patterns
+[What kept recurring? What does the month reveal about how PoC works?]
+
+## Links
+[Bidirectional links for the memory graph]
+- weekly digests → this monthly digest
+- this monthly digest → semantic keys
+
+## Looking ahead
+[What threads carry into next month? What's unfinished?]
+```
+
+Use ONLY keys from the semantic memory list below.
+
+---
+
+## Weekly digests for {{MONTH_LABEL}}
+
+{{DIGESTS}}
+
+---
+
+## Semantic memory nodes
+
+{{KEYS}}