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>

src/neuro.rs

@@ -189,14 +189,14 @@ pub fn schema_assimilation(store: &Store, key: &str) -> (f32, &'static str) {
 }
 
 /// Prompt template directory
-fn prompts_dir() -> std::path::PathBuf {
+pub(crate) fn prompts_dir() -> std::path::PathBuf {
     // Check for prompts relative to binary, then fall back to ~/poc/memory/prompts/
     let home = std::env::var("HOME").unwrap_or_default();
     std::path::PathBuf::from(home).join("poc/memory/prompts")
 }
 
 /// Load a prompt template, replacing {{PLACEHOLDER}} with data
-fn load_prompt(name: &str, replacements: &[(&str, &str)]) -> Result<String, String> {
+pub(crate) fn load_prompt(name: &str, replacements: &[(&str, &str)]) -> Result<String, String> {
     let path = prompts_dir().join(format!("{}.md", name));
     let mut content = std::fs::read_to_string(&path)
         .map_err(|e| format!("load prompt {}: {}", path.display(), e))?;