docs: replace generic code review instructions with repo rules

[locus313]

Summary

 .github/instructions/
-├── code-review-generic.instructions.md   # 418 lines, JS/Python examples, "customize for any project"
+├── code-review.instructions.md           # repo rules: block-merge and discuss lists

Follow-up to #38. Still applyTo: '**' with excludeAgent: ["coding-agent"], so only the review agent reads it.

block merge: credentials, deletes that can widen, %var% in batch, one-sided self-update edit, missing PID lock/umask
discuss:     parity gaps, docs not following the script, new script missing README/table row, glossary overlap, missing ADR

Evidence

• Before: the review agent got generic SOLID/N+1/React-style guidance that doesn't fit shell and batch scripts, and none of this repo's rules.
  After: a 25-line checklist that leaves shellcheck, drift and docs checks to CI and covers the judgement calls.

Merge Danger

Door: two-way

Blast Radius: Review agent

Copilot code review will flag different things. Rules are new and untested on real PRs, so expect to tune them.