feat: add registry validation diagnostics

[dapi]

Summary

• Add typed registry diagnostics with machine-readable codes, field/file paths, messages and recovery hints.
• Distinguish duplicate active records from conflicting active records for the same zellij pane.
• Add non-destructive file diagnosis coverage so invalid JSON leaves sessions.json unchanged.
• Document FT-008 selected diagnostic and recovery contract.

Validation

• go test ./...
• go vet ./...
• go build ./cmd/zelma
• go test -race ./...
• python3 scripts/check_memory_bank_index.py
• git diff --check
• rg --files memory-bank
• rg -n "zelima|Zelima" . (only existing guard/instruction references matched)

Closes #8