refactor: separate identity from chat agent, auto-load sol vars
Extract pure identity content from sol/identity.md (strip frontmatter,
<150 lines), create muse/chat.md as the new cogitate chat agent, and
add _load_sol_vars() to auto-load sol/*.md files as $sol_* template vars.
Update morning_briefing, pulse, heartbeat, and partner agents to use
$sol_identity instead of "system": "journal". Create muse/routines/SKILL.md
with template and command reference tables. Update agent routing so
"unified" resolves to muse/chat.