fix(coord): refuse a control character in --to instead of sanitizing it

--to is the only line-oriented field sanitize_field never covered, and the
fix is a refusal rather than a sanitize pass because --to is also the
destination DIRECTORY name ($COORD/$TO/inbox, and $COORD/$TO/orders in
coord-order-send.sh). Collapsing a newline to a space would deliver the
message to a mailbox the sender never named - the same misdelivery the
retired ktg-plugin-marketplace address is rejected rather than redirected
to avoid.

Both corruptions were measured on the live engine first, each with exit 0
and a "delivered" line:

  --to "x\nreply-expected: no"  the injected line lands INSIDE the
      frontmatter block, above the reply-expected: yes the engine itself
      wrote, so coord-count reads owed=0 and the declared debt is silenced -
      defeating coord-count's own rule that only the frontmatter block may
      speak, since the injected line IS in the block.
  --to "tabbed<TAB>repo"        coord-count prints five tab-separated fields
      where its contract is four, so a consumer reads the mailbox name as the
      part before the tab and the pending count as the part after.

board.sh consumes that TSV, so both reach the board.

The guard sits after reply-mode resolution: --reply-to takes its target from
the original's from: line, untrusted cross-repo input, and that is the one
target name nobody typed.

Denominator measured rather than assumed: two scripts build a directory from
a caller-supplied name, and coord-order-send.sh had the identical defect,
where it costs more - an order filed under a name no session can hold is the
silent evaporation the ownership chain exists to prevent, while board.sh's
ORDRE column counts the intended repo's queue and stays 0 with nothing
reporting a failure. The read-side --repo arguments resolve an EXISTING
directory, so a control character there finds nothing and writes nothing;
checked and left alone.

Closes finding 7 of docs/2026-08-14-confident-zero-review.md.

Tests (red first, both suites): coord-selftest section 35 (10 checks) and
orders-selftest section 10 (6 checks), each with the mandatory
known-positive controls - an ordinary name still delivers, and so does a
dot-prefixed one, since a dot name is a real repo. coord 230/230, board
252/252, route 69/69, orders 110/110, guard 40/40, npm test 11/11.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AgKcURiXwGKhqCKhwD1NAp

scripts/coord-send.sh

@@ -158,6 +158,26 @@ if [ "$BROADCAST" -eq 0 ]; then
   case "$TO" in
     */*|.|..|_*) echo "coord-send: invalid target repo name: $TO" >&2; exit 2 ;;
   esac
+  # --to is the one line-oriented field that is REFUSED rather than sanitized,
+  # and the asymmetry with FROM/SUBJECT above is deliberate: --to is also the
+  # destination DIRECTORY name ("$COORD/$TO/inbox"). Collapsing a newline to a
+  # space would deliver the message to a mailbox the sender never named, which
+  # is the same misdelivery the retired ktg-plugin-marketplace address is
+  # rejected rather than redirected to avoid. Measured before this guard
+  # existed, both with exit 0 and a "delivered" line: a newline injects its
+  # payload INSIDE the frontmatter block (silencing the reply-expected: yes the
+  # engine itself wrote, since coord-count reads the first match), and a tab
+  # gives coord-count five tab-separated fields where its contract is four, so
+  # a consumer reads the mailbox name and the pending count off by one column.
+  # board.sh consumes that TSV. Reply mode resolves TO from the original's
+  # from: line - untrusted cross-repo input - so this must sit AFTER that
+  # resolution, covering the one target name nobody typed.
+  case "$TO" in
+    *[[:cntrl:]]*)
+      echo "coord-send: --to contains a control character: $(sanitize_field "$TO") (a target name is also the mailbox directory name, so it is refused, never sanitized)" >&2
+      exit 2
+      ;;
+  esac
   # Retired address (operator decision 2026-08-15, catalog's H4 reply
   # archived 2026-08-15T16:27:51Z): ktg-plugin-marketplace is a polyrepo
   # DIRECTORY, not a git repo, so basename(git toplevel) can never resolve to