* feat(usage): add Muse Code local usage provider
Scan Muse session logs (including subagent logs, which hold usage the parent
log does not) for model_completed token events and surface them as a fourth
local usage provider: shared scan worker, persisted per-file cache reused by
mtime/size, cross-log dedupe, Stats tab, and Usage Overview integration.
Muse logs carry no price, so the provider reports tokens only.
* fix(usage): name Muse in Stats & Usage copy; skip partial-cost warning when nothing is priced
* fix(usage): surface unreadable Muse sessions root; name Muse in remaining Stats & Usage copy
* fix(usage): count distinct same-content Muse records within one log
Neil