docs(agents): adopt the family no-em-dash prose rule (convention 14)

AGENTS.md

@@ -154,6 +154,21 @@ session, not just spawned implementation agents.
     the feature graph is independent and flat until architecture says
     otherwise.
 
+14. **No em dashes in prose.** Do not use the em dash (U+2014) in source
+    doc comments, markdown docs (`docs/`, `README.md`, `CHANGELOG.md`,
+    `AGENTS.md`), agent prompts under `.opencode/agents/`, or commit
+    messages. Restructure instead: split the sentence, use parentheses,
+    commas, a colon, or a semicolon, whichever reads
+    naturally, and vary the choice rather than converting every instance
+    the same way. A spaced hyphen (` - `) is not an acceptable
+    substitute; it reads as a different but equally recognizable writing
+    tell. The same rule covers en dashes used as clause separators,
+    though en dashes inside numeric ranges keep their normal meaning.
+    Hyphens in compound words (`fail-closed`, `client-side`) are
+    untouched. In test fixtures and scripts the rule applies to prose-y
+    content only: comment lines are rewritten too, while fixture data,
+    wire examples, and tool transcripts survive verbatim.
+
 ## Verification Commands
 
 Run these before committing. All must pass.
@@ -164,6 +179,9 @@ cargo clippy --all-targets -- -D warnings
 cargo fmt --check
 cargo doc --no-deps                                 # if docs changed
 cargo publish --dry-run --allow-dirty               # before a release
+grep -rn '—' src/ docs/ README.md CHANGELOG.md AGENTS.md \
+    .opencode/agents/ tests/ scripts/               # must be zero hits;
+                                                    # see convention 14
 scripts/matrix.sh                                   # the decline-matrix runner: the full suite per
                                                     # feature build (none / git / session / tunnel /
                                                     # sftp / --all-features), clippy ×2, fmt, doc