refactor: move day content under journal/chronicle/
Relative paths stay chronicle-free; the prefix is applied at the
rel↔abs boundary via resolve_journal_path(), CHRONICLE_DIR, and the
updated day_path()/day_dirs(). Indexer, formatters, hooks, and merge
thread through the new boundary. apps/sol/maint/002_migrate_chronicle.py
relocates existing day dirs on next startup and forces a clean
reindex; merge.py is the only production code that reads both layouts
(for external archives). All ~67 fixture day dirs moved via git mv.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>