planning: cap path-hit output in check-plan-outcome.sh

[kyle-sexton]

Part of #3615; found by the #5291 planning scripts audit.

Problem

plugins/planning/scripts/check-plan-outcome.sh:245 prints one path-hit=<line>:<text> line per non-portable path with no upper bound, so a PLAN.md with hundreds of such lines produces unbounded stdout (checklist item: output-size truncation limits).

Work

• Print at most N hits (for example 20) followed by path-hit-truncated=<remaining>; keep the criterion=portable-paths status=fail line and exit code unchanged.
• Update the header contract (check-plan-outcome.sh:49-52) and add a case to check-plan-outcome.test.sh.
• Observable change: consumers that count path-hit= lines (grep in /planning:plan Step 4.7 and its tests) must be checked first.

Size: S.