CI has no smart segmentation for docs-only PRs — full fat job + mandatory pr-review quorum run regardless of change type #4472

What

CI has no "docs-only" classification that scales down to the checks a docs-only diff
actually needs. Every PR — including one that touches only README.md and two
book/src/**.md pages, zero Rust, zero config, zero CI/workflow files — pays for
close to the full verification surface: the fat x86-main job (mutation testing, the
sov.* matrix, the determinism raster, both pr-review-shadow/pr-review-sign),
mac-check, determinism, full book chapter-compile, and the mandatory pr-review
quorum receipt (which itself requires a real pmat index build plus a mandatory
cross-vendor agy consultation, exactly as heavy as it would be for a CUDA kernel
rewrite).

This is not a request to remove verification for docs PRs — a docs change can still
ship a false claim (docs/BEATS.md, README's own count-drift class of bug this repo
has hit before) or break a contract-gated book page. The request is that the
checks a diff actually cannot affect should not run for it, the same principle
scripts/ci_test_tier.sh already states for Rust compilation but does not apply
consistently, and does not apply at all to the pr-review quorum.

Concrete evidence: PR #4467

PR #4467 (this repo) changes exactly 3 files: README.md,
book/src/introduction.md, book/src/getting-started/installation.md — adding one
curl | sh install line to each, under existing headings. No Rust file, no
Cargo.toml, no CI workflow, no contract YAML is touched.

What ran anyway (gh pr checks 4467):

scripts/ci_test_tier.sh on this exact diff (I ran it by hand):

tier=quick
crates=
targets=<136 tree-reader test targets across ~40 crates>
reason=pull_request: no workspace crate touched -> nothing to test;
       plus 136 tree-reader target(s) from scripts/tree_reader_tests.txt

So the one piece of this repository's CI that does already reason about "no crate
was touched" still resolves to 136 test targets spread across ~40 crates, because
each one is registered as reading some part of the tree and the registry doesn't
distinguish "reads README.md" from "reads src/**.rs". That's a real, if partial,
cost reduction (it is not the ~80k-test full workspace run) — but it is not what a
docs-only diff should cost, and it says nothing about the mutation sweep, the sov.*
matrix, the determinism raster, or the pr-review-sign/pr-review-shadow jobs, which
run unconditionally regardless of tier.

What already exists (so this is an extension, not new infrastructure)

• scripts/ci_test_tier.sh already has a tier=none fast path for diffs where every
  touched path is under docs/roadmaps/ or docs/audits/ — but that allowlist does
  not include README.md, book/src/**, or top-level *.md files, so this PR's
  actual diff doesn't qualify for it.
• scripts/check_pr_review_receipt.sh's own --match-* predicates already correctly
  identify a docs-only diff as not-triggering CUDA/CRUX/mutation (I verified this by
  hand for PR ci: publish scripts/install.sh to paiml.com/apr/ via CloudFront/S3 #4461, structurally the same shape as docs: advertise curl | sh install via paiml.com/apr (README + book) #4467) — but "not-triggered" for
  those three consultations does not reduce what's mandatory: pmat and the
  cross-vendor agy run are required on every PR regardless, at full weight.
• Neither mechanism talks to the other. There is no single, shared "what kind of
  change is this" classifier that both ci_test_tier.sh and the pr-review guard
  consult — each has independently reinvented a narrower slice of the same idea.

Proposed direction (not a design to implement as-is — needs its own review)

1. One shared classifier, not two independent ones. A diff's classification
   (docs / ci-config / source / mixed) should be computed once (e.g. as a
   ci_test_tier.sh-adjacent script, or an extension of it) and consumed by both the
   fat job's section list and the pr-review guard, rather than each maintaining its
   own path patterns that can and do drift apart.
2. Widen the existing tier=none/docs allowlist to cover the paths that are
   actually docs by any reasonable definition — README.md, book/src/**,
   docs/** broadly (not just roadmaps/+audits/), root-level *.md — while
   still running the specific checks that verify doc content itself
   (readme_contract, book_contracts, the per-page pv validate contract checks,
   link/anchor checks) rather than the unrelated 136-target tree-reader sweep.
3. Make the fat job's non-source sections tier-aware. mutants (mutation
   testing) and the sov.* matrix have no meaningful signal to produce over a diff
   that adds two lines of Markdown; they should be skippable (reported as
   not-triggered, not silently green) for a docs classification, the same honesty
   discipline the pr-review skill already applies to its own consultations (§3.0:
   "not-triggered" is a distinct, visible state from "consulted, found nothing" —
   apply that discipline to the fat job's sections too).
4. Give pr-review quorum a genuinely lighter receipt tier for a docs
   classification — not zero verification, but proportionate:
   • Keep: the diff-patch-id binding + signature (integrity/provenance still
     matters regardless of diff size), the pmat duplication/trigger checks (already
     fast once the index is warm), and the existing CUDA/CRUX/mutation trigger checks.
   • Make optional: the mandatory cross-vendor agy consultation, which is
     genuinely expensive (a disposable git-archive tree, several minutes of wall
     time, real token cost) and adds ~nothing over a diff that adds an install
     instruction to three Markdown files, where the CUDA/CRUX/mutation triggers
     already independently confirm nothing user-facing, GPU-related, or
     comparative-claim-bearing changed.
5. Whatever a docs classification skips must be recorded as not-triggered with a
   named reason, never silently absent — this repo's own pr-review skill already
   states the principle that governs this (§3.0): "not-triggered" and "could not
   consult" must never read the same as "consulted, found nothing," and the same
   applies one level up, to entire jobs/sections being skipped for a diff
   classification rather than for a failure.

Non-goals

• Not proposing docs PRs skip verification of doc content — readme_contract,
  book_contracts, and the per-page contract checks all caught real problems before
  and should keep running (in fact should probably run for MORE doc paths than they
  currently do narrowly, not fewer).
• Not proposing a source-touching PR (however small) get a lighter tier — the
  existing rule (ii) (root manifest touched -> full) and the crate-cap rule in
  gate_touched_crates.sh should stay exactly as strict as they are.
• Not proposing an implementation here — this crosses ci_test_tier.sh,
  ci/sections.yml's fat-job section list, and contracts/pr-review-skill-v2.yaml's
  §3 triggers, and deserves its own design pass (particularly picking the shared
  classifier's exact path rules) before anyone writes the workflow/script changes.