docs(dogfood): step 4 requires rebuilding before judging the ablation - #8821

Fixes #8366

What

packages/qa/dogfood/README.md, "Adding a golden test" step 4, prescribed the
ablation procedure (revert the fix, confirm the test goes red) without the
rebuild in between. This suite resolves the code under test from each
package's built dist/, not src/, so a revert without a rebuild leaves the
ablation running against the pre-mutation build and staying green — a vacuous
test silently certified as discriminating, with no later CI run able to
expose it (CI builds correctly, so it stays green there forever).

This is the third live copy of the same procedure. The other two —
.claude/agents/os-dev.md and .claude/skills/dogfood-verification/SKILL.md
— already carry the rebuild step and the pre-flight script reference, landed
by PR #8365. This PR brings the README copy in line with that landed wording,
plus one extra sentence of why since this file is the copy a human
contributor is most likely to read first.

Change

One clause added to step 4:

• rebuild the package (pnpm --filter PKG build) between reverting the fix
  and judging the test's colour;
• reference scripts/ablation-dist-preflight.mjs PKG 'MARKER' (--absent
  for a deleted guard) as the pre-flight that proves the mutation actually
  reached the built artifact.

(The README itself uses angle-bracket placeholders; they are spelled PKG /
MARKER here because this description is stored through a sanitizer that
strips angle-bracket sequences — an earlier revision of this body lost them
silently. The committed file is unaffected; see the diff.)

Verified the script's real path/name/invocation against origin/main before
citing it (scripts/ablation-dist-preflight.mjs, confirmed present and
matching the card's citation exactly — no discrepancy). Also read back both
landed copies (.claude/agents/os-dev.md:229-237,
.claude/skills/dogfood-verification/SKILL.md:64-75) to confirm they agree
with each other and with what this PR now adds to the README — no
three-way drift.

Docs-only, no runtime/behavior change. No .changeset/ entry (nothing
user-visible); skip-changeset applied.

Refs #8246 (closed, discharged the body's Blocked-by: — not addressed
here), PR #8365.

Tests

Docs-only change; no build/test suite applies. Local gates run at the final
commit (ab48e32ce, from node scripts/pm/dispatch-gates.mjs packages/qa/dogfood/README.md, plus check:nul-bytes on every edit):

check-nul-bytes: OK (scanned 5838 text file(s) -- 5838 tracked, 0 untracked-not-ignored; skipped 5 binary; no raw ASCII control bytes).
check-test-source-alias OK — 72 packages with tests scanned; 61 registered as still resolving a workspace dep through `dist/`.
check-type-source-resolution OK — 76 packages with a tsconfig.json scanned; 51 registered as still resolving a workspace dep's types through `dist/`.