Resolve conflicts:
- secheader.go: keep both HEAD ppe header and main AgentTrace header
- proxy_test.go: union of both test sets; align main's plugin tests to the
2-arg WarnIfProxied(w, interactive) signature
- SKILL.md: take feat side (refactored 2.0.0 + others' value-render-option removal)
- drop 5 legacy sheets sources + cell-data.md (refactored away; main's
#996/#1001/#984/#1073 covered by new code or no longer applicable)