Daily cost fix for custom session storage
- The daily cost tracker now scans every session storage root: the built-in store, the session's own flat root (
--session-dir, extension hook or settings), the global and effective sessionDir, and the configured and legacy subagentSessionDir roots. Before this, custom storage roots were left out of "today".
- A file that can be reached from more than one root is counted once.
- The TUI footer and the
get_daily_cost RPC (Dashboard) both use the same root list.
Implemented in PR 529, resolving issue 528.
