docs(implementation): merge backend reference docs into implementation summaries

Each of the four backend pairs (Agda, CVC5, Z3, Mizar) had a user-facing
docs/<NAME>_BACKEND.md and an internal docs/implementation/<NAME>_IMPLEMENTATION_SUMMARY.md.
The audit found significant overlap and divergent staleness. Per the
agreed direction (docs/implementation/ as canonical), each <NAME>_BACKEND.md
is now appended as a clearly-labelled "Appendix: <Name> Backend Reference"
section in the corresponding _IMPLEMENTATION_SUMMARY.md, then deleted.

The appendix carries a note that a future polish pass should integrate
the user-facing material naturally into the summary structure rather
than leaving it siloed.

Quick-start / quick-reference docs (CVC5_QUICK_REFERENCE, MIZAR_QUICK_START,
ASPECT_QUICK_START) remain separate — they serve a distinct purpose.

https://claude.ai/code/session_01YPqu7gti4azBach6ZvpRFJ

Author: claude