Per approved spec in #1969, the planner must include CONTEXT.md and
SUMMARY.md from any phases listed in the current phase's 'Depends on:'
field in ROADMAP.md, in addition to the 3 most recent completed phases.
This ensures explicit dependencies are always visible to the planner
regardless of recency — e.g., Phase 7 declaring 'Depends on: Phase 2'
always sees Phase 2's context, not just when Phase 2 is among the 3
most recent.
Review feedback on #2058 from @trek-e.
e0b766a08b