docs: surface CI pass requirements earlier in README

[bonaventure001]

Context

While working on #142, I found that comprehensive CI pass requirement
guidance already exists in this repo:
docs/failing-ci-guide.md — covering failure categories (Rust
compilation, Soroban, Makefile, dependency, workflow dispatch), local
reproduction commands, and maintainer expectations, and it's already
linked from the README.

Rather than duplicate that work, this PR addresses a real, smaller gap:
discoverability. The only README link to the guide was buried far
down the page, and the Testing section (the natural first stop for a
contributor about to push) had no pointer to it at all.

Changes

• Added a note in the Testing section pointing to make verify and the
  Failing CI Response Guide, visible before a contributor pushes
• Added a quick-reference failure-type table in the CI & Contributing
  section for fast scanning, alongside the existing detailed prose guide

Acceptance criteria mapping

• CI pass requirement guidance exists — already present in
  docs/failing-ci-guide.md (pre-existing)
• States failing CI can affect evaluation — already present
• Common failure types listed — already present in prose; this PR
  adds a scannable table version
• Local reproduction commands included — already present
• Contributor fix expectations documented — already present
• README links to the guidance — already present at one location;
  this PR adds a second, earlier pointer for discoverability

Note for reviewers

This is intentionally a small, honest PR. The substantive guidance was
already built; I did not want to pad this out or duplicate existing
content just to appear more substantial. Happy to expand scope if a
maintainer identifies a genuine remaining gap.