docs: Remove completed sprint plans and continuation prompts

Deleted obsolete planning documents:
- sprint-1-plan.md (Phase 1 complete)
- conditionals-sprint-plan.md (Phase 2 complete)
- wave-4-completion-report.md (Sprint 1 Wave 4 report)
- session-handoff-test-fixes.md (continuation prompt)

All sprint progress tracked in master-checklist.md

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

Author: randlee