Sub-agent cost tracking
- The daily cost tracker now includes sub-agent sessions (direct and chain-agent
step-N layouts), so "today" no longer undercounts delegated spend.
- The TUI footer and Dashboard show a per-session breakdown:
$main + $subs, today: $total. It refreshes when a background agent finishes and still appears when the parent session's own cost is zero.
- The
get_daily_cost RPC now returns main and subagent alongside cost, and get_session_stats includes subagentCost.
- Dashboard performance: live runtime cards and SSE load before the historical session inventory, and session metadata is cached by file mtime and size (custom flat session directories included).
Implemented in PR 527, resolving issue 525.
