docs(follow-forks): add process-tree-following plan + filter.c reference

Document the planned opt-in "follow forks" mode that would let ior trace a
target PID and all its descendants (needed for the landlock_restrict_self
integration case, task ci0, and for tracing forking workloads as a tree).
The plan covers the BPF descendant-set map, sched_process_fork/exit hooks,
the FOLLOW_FORK gate in filter(), userland flag/seeding/assertion changes,
and explicitly requires syscall-count aggregation to roll up across the
followed tree. Add a reference comment above filter() pointing to the plan.

Plan only — not implemented.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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