The original build_track_index spawned 5+ git subprocesses per track
folder (244 tracks * 5 calls = 1200+ subprocesses). That exceeded the
120s NFR1 budget. Replaced with a single 'git log --all --name-only'
pass + a mutable _TrackAccumulator; reduces git subprocess count from
1200+ to 1.
Also adds 3 smoke tests for build_subagent_task_index,
build_session_index, build_track_index (covering previously untested
pure-function indexers). 23 tests pass; coverage up from 39% to 61%.
ed
96c5b94616