docs: blog edits

blog: restore plot images + fix unclosed code span in reward-hacking table

blog: add reward-hacking section (3 patched exploits + 3 by-construction invariants)

docs: drop per-component reward breakdown chart and bullets

blog: clean pass + future scope section

blog: only 3 confirmed trained systems from W&B

blog: show all 6 systems honestly; GRPO=3, SFT=6, no held-out claims

blog: expand system tiers table, add extensibility section

Add blog.md: accurate systems table, SFT+GRPO plots