refactor(filtering): consolidate task list filters into single strategy registry

Replace the three drifting filter implementations (counter, list loader,
regroup) with one ITaskListFilter strategy per list kind. Counter and list
loader now share the same predicates, so they cannot diverge again. Planning
hierarchy rules (parent-as-context, orphan handling) live in PlanningRules
and are unit-tested via 29 new tests in ClaudeDo.Data.Tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/ClaudeDo.Data.Tests/ClaudeDo.Data.Tests.csproj

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