tests/test_session_lineage_collapse.py invokes 'node -e <source>' where
<source> embeds the entire static/sessions.js content. Linux's
MAX_ARG_STRLEN is 131,072 bytes per argv arg; sessions.js plus the test
scaffolding now exceeds that limit, producing OSError(Argument list too
long).
Switching to 'node' with source via stdin removes the limit. No behavioral
change to the tests themselves — they still exercise the same JS functions
on the same input data.
When a session's compression lineage spans multiple segments (linked via _lineage_root_id from api/agent_sessions.py), the sidebar previously rendered each segment as a separate top-level row. Adds _collapseSessionLineageForSidebar() that groups by lineage root and keeps only the most recently active tip per group, with a _lineage_collapsed_count marker for future UI affordances.
Co-authored-by: Dennis Soong <dso2ng@gmail.com>
