feat(write-path): the derive arm — the hint names a duplicate family (no canon) or a canon elsewhere for the shapes being written; exclude_derive channel (#2900, milestone 299 step 2)

shape_ledger.write_time_derive asks the ledger what it knows about each
named (kind, symbol): a derive-grouped family (identical body / same name
in N other files) -> "derive it now, do not add a copy"; a canonical row
at another path -> "canon #N at <path>, reuse". Judged rows at the path and
the canon own file stay silent. Rendered by _derive_line beside the
divergence line; keyed (group id / canon:<id>) on a third per-session dedup
channel in the hook (.derive.ids -> exclude_derive=).

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

plugin/hooks/scribe_prior_art.sh

@@ -258,14 +258,22 @@ fi
 # the sync nudge when the recorded file itself is edited later.
 state_dir="${TMPDIR:-/tmp}/scribe-priorart"
 mkdir -p "$state_dir" 2>/dev/null || true
+#
+# A THIRD channel (#2900): the ledger's derive arm names a duplicate family
+# (a derive group id) or a canon elsewhere (`canon:<snippet_id>`) for the
+# shapes being written. Keyed by that token, not a note id, so it dedups on
+# its own file and a family is named once per session, not at every edit.
 idfile=""
 syncfile=""
+derivefile=""
 exclude_q=""
 sync_exclude_q=""
+derive_exclude_q=""
 if [ -n "$session_id" ]; then
   safe_sid=$(printf '%s' "$session_id" | tr -c 'A-Za-z0-9._-' '_')
   idfile="$state_dir/${safe_sid}.ids"
   syncfile="$state_dir/${safe_sid}.sync.ids"
+  derivefile="$state_dir/${safe_sid}.derive.ids"
   if [ -f "$idfile" ]; then
     seen=$(tr '\n' ',' < "$idfile" 2>/dev/null | sed 's/,$//')
     [ -n "$seen" ] && exclude_q="&exclude_ids=${seen}"
@@ -274,13 +282,17 @@ if [ -n "$session_id" ]; then
     sync_seen=$(tr '\n' ',' < "$syncfile" 2>/dev/null | sed 's/,$//')
     [ -n "$sync_seen" ] && sync_exclude_q="&exclude_sync_ids=${sync_seen}"
   fi
+  if [ -f "$derivefile" ]; then
+    derive_seen=$(tr '\n' ',' < "$derivefile" 2>/dev/null | sed 's/,$//' | jq -sRr '@uri' 2>/dev/null) || derive_seen=""
+    [ -n "$derive_seen" ] && derive_exclude_q="&exclude_derive=${derive_seen}"
+  fi
 fi
 
 # `|| true`, not `|| exit 0`: an unreachable instance must not discard a local
 # finding that needed no instance to produce.
 body=$(curl -fsS --max-time 5 \
   -H "Authorization: Bearer ${token}" \
-  "${url%/}/api/plugin/prior-art?path=${path_enc}&code=${code_enc}${repo_q}${exclude_q}${sync_exclude_q}${shapes_q}" 2>/dev/null) || body=""
+  "${url%/}/api/plugin/prior-art?path=${path_enc}&code=${code_enc}${repo_q}${exclude_q}${sync_exclude_q}${derive_exclude_q}${shapes_q}" 2>/dev/null) || body=""
 
 context=""
 if [ -n "$body" ]; then
@@ -295,6 +307,9 @@ if [ -n "$body" ]; then
     if [ -n "$syncfile" ]; then
       printf '%s' "$body" | jq -r '(.sync_note_ids // [])[]?' 2>/dev/null >> "$syncfile" || true
     fi
+    if [ -n "$derivefile" ]; then
+      printf '%s' "$body" | jq -r '(.derive_keys // [])[]?' 2>/dev/null >> "$derivefile" || true
+    fi
   fi
 fi