test(#1074): swap workflow size enforcement to baseline + loose hard caps (PR 2/3) (#1096)

Completes the #1074 migration for workflows. The per-file baseline (PR 1) is
now the primary anti-creep guard, so the tier-max tighten-only ceilings are
retired here.

- Delete the 'SIZE: tier anti-creep' describe (3 assertTightCeiling tests) and
  the GRACE constant — per-file baseline already guards every file by name,
  strictly stronger than max(tier).
- Convert the per-file tier test into 'SIZE: workflow tier hard caps': absolute
  red lines (XL 96 KiB / LARGE 60 KiB / DEFAULT 40 KiB) that mean 'extract, do
  not raise', each with real headroom above its high-water file.
- Add a 32 KiB (Codex project_doc_max_bytes) cap for net-new workflow files not
  yet in the baseline and not explicitly tiered.
- Rewrite the header doc comment for the new two-guard model; drop the
  assertTightCeiling import (now unused here; still exported + used by the agent
  test until PR 3).

Addresses the three PR-2 items from the #1089 review:
- Finish the enumeration consolidation (Minor #1): the tier hard-cap and
  new-file guards now read both their file list and byte sizes from
  measureWorkflows()/listWorkflowStems(), removing the inline readdirSync +
  per-file byteCount split-brain. Enumeration and measurement share one source.
- Rewrite CONTEXT.md RULESET.WORKFLOW_SIZE_BUDGET (Minor #2): stale caps
  (XL<=90000/LARGE<=54000/DEFAULT<=38000) replaced with the baseline-first
  model, current hard caps, the new-file anchor, and the size:baseline
  remediation.
- Ship docs (contract-change requirement): a Diataxis how-to + reference for
  the size guard in docs/TESTING-SUITES.md, beside the sibling regression-name
  ratchet (the 'file grew, CI red -> npm run size:baseline, commit the one-line
  diff, justify or extract lazily' workflow). CONTRIBUTING.md's workflow-tree
  note is updated to the baseline model and points at the new guide.

Negative proof: each guard fails independently when violated (new-file cap at
33 KB; hard cap with baseline current; baseline on any per-file growth).

Refs #1074. Part 2 of 3.

CONTRIBUTING.md

@@ -850,8 +850,15 @@ gsd-core/
                           the canonical example (#2551). New modes for
                           discuss-phase land in
                           workflows/discuss-phase/modes/<mode>.md.
-                          Per-file budgets enforced by
-                          tests/workflow-size-budget.test.cjs.
+                          Per-file sizes are pinned by a committed baseline
+                          (tests/workflow-size-baseline.json) plus loose tier
+                          hard caps, both in tests/workflow-size-budget.test.cjs.
+                          If you legitimately grow or shrink a workflow file,
+                          run `npm run size:baseline` to update the snapshot and
+                          justify any growth in your PR (or extract content
+                          lazily). Full how-to + reference in
+                          docs/TESTING-SUITES.md (Workflow size budget); see
+                          issue #1074.
   references/           — Reference documentation (.md)
   templates/            — File templates
 agents/                 — Agent definitions (.md) — CANONICAL SOURCE