docs(subtract): correct the ground-truth counts and keep the gate re-runnable

Two record fixes, no behaviour change.

The README quoted "35 blocks, 13 genuinely ambiguous" for the hand-built ground
truth. Counting its own rows gives 48 classified blocks and 19 ambiguous — the
headline in the fasit disagreed with the table beneath it, apparently by
collapsing letter-suffixed sub-blocks for the summary while listing them
separately. Labels are untouched in both files; only the counts are restated,
and now as row counts, which are reproducible with a grep rather than by
recounting a classification.

The dogfood gate script that proves the blocking §8 criterion lived only in the
session scratchpad, which would have made "zero load-bearing blocks proposed,
11/18 groups, ~756 tok" unverifiable claims the moment the session ended —
precisely the premise-not-fact class this repo's own rules warn about. It now
lives at scripts/dogfood-subtraction-gate.local.mjs, gitignored via a new
*.local.mjs pattern because it indexes the operator's private config by line
number and must never reach the public mirror.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RW2haJXbxZpKivKHseSXNh

.gitignore

@@ -38,5 +38,8 @@ NEXT-SESSION-PROMPT*.local.md
 *.local.md
 *.local.json
 *.local.sh
+# Local-only dogfood harnesses: they index the operator's private config by line
+# number (see docs/subtraction-fasit.local.md) and must never reach the public mirror.
+*.local.mjs
 .DS_Store
 .claude/