docs(agents): practise what the line-width note preaches

The note telling you not to hard-wrap prose was hard-wrapped across
two paragraphs and 19 lines, having been offered as one. Now one
unwrapped line, which is both the convention it states and the length
it promised.

Dropped in the trim: the per-medium enumeration of what makes no
difference (rendering, build, size, tokens), the blame-commit counts,
the .git-blame-ignore-revs escape hatch, and the exception's field
list. The measurements stay in this PR's description and #386's
history; a convention needs the rule and the one number that justifies
it, not the workings.

Note the section it sits in was already wrapped while "2.0 API
modules" below is not, so the file was mixed before either note
existed -- which is why the §5 bullet in the previous commit is
unwrapped and correct without anyone deciding.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: derek73

AGENTS.md

@@ -200,25 +200,7 @@ the two clears the words-to-spare guard. Running axis 1 over this
 very list caught an earlier wording of that sentence conflating the
 population with the movers, so the count above is the corrected one.
 
-**Markdown line width: don't hard-wrap prose.** Measured 2026-08-16,
-the ~80-column wrap in these files buys nothing. Wrapped and unwrapped
-markdown render identically (single newlines inside a paragraph
-collapse to spaces), docs/design/ is not in the Sphinx build, and
-unwrapping is byte-identical because newline→space is 1:1 — so there
-is no rendering, build, size or token argument either way. The cost is
-one-sided: 74% of the deleted words in #386's decisions.md diff were
-reflow rather than change, which is churn a reviewer has to read past.
-Write new prose unwrapped and leave existing paragraphs until they are
-being rewritten anyway — a bulk rewrap would reset `git blame` across
-the 31 and 43 authoring commits in rules.md and decisions.md, which is
-the provenance that dates a stale claim (do it as one reformat commit
-in .git-blame-ignore-revs if you ever want it uniform).
-
-The exception is enforced, not stylistic: rules.md's example lines and
-its `history:`/`interacts:`/`implemented:`/`tracked:` pointer lines are
-parsed per line, and a wrapped example line is a hard error naming its
-rule. Rule STATEMENT prose is free to rewrap, because the citation
-check normalizes whitespace before comparing (`" ".join(s.split())`).
+**Markdown line width: don't hard-wrap prose.** Measured 2026-08-16 — wrapped and unwrapped render identically, docs/design/ is not in the Sphinx build, and unwrapping is byte-identical (newline→space is 1:1), so the width buys nothing and costs diff churn (74% of #386's decisions.md deletions were reflow, not change). Write new prose unwrapped; don't bulk-rewrap old paragraphs, which would reset `git blame` on the provenance that dates stale claims. Enforced exception: rules.md's example and pointer lines are parsed per line, so a wrapped example is a hard error naming its rule — statement prose is free, since the citation check normalizes whitespace first.
 
 **Guard tests** SHOULD carry a recorded negative control — the
 answer with the guard off, stored as data (the _EXCLUSION_EFFECT