Harden self-conformance into a real gate

What does this MR do and why?

The self-conformance job ran with allow_failure: true, under a comment calling conformance an advisory reminder. Dropping it costs nothing today and catches a regression tomorrow. The five advisory-by-contract standards (design, llms-txt, posture-claims, presentation, upkeep) declare their tier inside their own check.sh, and _lib.sh's _fail() routes a tripped assertion in an advisory standard to ADVISE and never to _FAIL, so only a genuinely blocking standard can fail this job.

This MR's own pipeline is the demonstration. On this branch just check reports 21 standards passed, 0 failed, while still emitting one ADVISE for the two files that load mermaid from a CDN. The gate goes green with an advisory finding outstanding, which is the property the change relies on.

References

The advisory routing this depends on is _lib.sh's _fail() and each standard's contract line.

Standards affected

None. No standard's assertions or tier change; this is the CI job that runs them.

Exemption implications

None.

Test plan

• just check on this branch: 21 standards passed, 0 failed, exit 0, with one ADVISE present
• just check at origin/main before the change: 21 passed, 0 failed, so nothing is newly blocking
• just guard, just validate, just build && just agent green