When CONTEXT_WINDOW >= 500000 (1M models), the planner loaded ALL prior
phase CONTEXT.md and SUMMARY.md files for cross-phase consistency. On
projects with 20+ phases, this consumed significant context budget with
diminishing returns — decisions from phase 2 are rarely relevant to
phase 22.
Limit to the 3 most recent completed phases, which provides enough
cross-phase context for consistency while keeping the planner's context
budget focused on the current phase's plans.
Closes#1969
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>