scripts: forge write-back — close the git→gitea→agentic loop

The forge already DRIVES sessions (briefing → tmux launch, sx-fix-up.sh).
This records what comes BACK, making the forge a true system of record:

- sx-fix-writeback.sh <forge-agent> [kind] [base-ref]: reads new commits on
  loops/sx-<slug>, appends a record per commit to writeback.sxsrc (idempotent,
  matched by sha), then rebuilds the forge + replays them as agentic-sx
  commit!s on agents/<forge-agent> and re-dumps forge.sxdata.
- forge-build.sxsrc: fb-writeback-records / fb-replay-writeback / fb-do-writeback
  — each real-git commit becomes an agentic-sx commit whose tree is a small
  commit.sx pointer (sha/branch/message/files); real git holds the code, the
  forge holds the index, so the CID stays small.
- writeback.sxsrc: the append-only record log (source of truth for what's
  been recorded); replayed chronologically so agent branch heads advance right.

Verified live: the sx-gate loop's first real commit (f09368e1, "pin K18
expt-overflow float-promotion") is now recorded as a test-kind agentic-sx
commit on agents/ws-W14 (session log: spawn → finding → writeback), its
commit.sx pointing back at the real-git sha.

Loop closed: forge → tmux (drive) and tmux → real-git → forge (record).

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

plans/sx-review/forge-build.sxsrc

@@ -103,6 +103,27 @@
       "(starting-point :real-git \"8651cefe\" :tree \"plan+rulings+forge+quick-wins\" :suite \"5762p-274f-baseline\" :next \"W14 test-gate first, then W1 K01 guard-hang\" :rule \"no semantic fix before its pinning test + gate\")")
     {:message "START — fix work begins from 8651cefe; W14 gate first, then W1/K01"}))
 
+; --- WRITE-BACK: real-git commits recorded into the forge as agentic-sx commits ---
+; Closes the loop: the forge drives the tmux session (briefing -> launch) AND
+; records the work that comes back. fb-writeback-records is overridden by loading
+; plans/sx-review/writeback.sxsrc (maintained by scripts/sx-fix-writeback.sh);
+; each record routes a real-git commit onto its forge agent branch. The commit's
+; tree is a single commit.sx pointer (sha+message+files) — real git holds the code,
+; the forge holds the index, so the CID stays small.
+(define fb-writeback-records (list))
+(define fb-replay-writeback
+  (fn (rec)
+    (agentic/commit! fb-sp (get rec :agent) (get rec :kind)
+      (assoc {} "commit.sx"
+        (str "(git-commit :sha \"" (get rec :sha) "\" :branch \"" (get rec :branch)
+             "\" :message \"" (get rec :message) "\" :files \"" (get rec :files) "\")"))
+      {:message (str "writeback " (get rec :sha) ": " (get rec :message))})))
+(define fb-writeback-cids (list))
+(define fb-do-writeback
+  (fn () (begin
+    (set! fb-writeback-cids (map fb-replay-writeback fb-writeback-records))
+    fb-writeback-cids)))
+
 ; --- durable dump: kv + streams of the shared backend, serialized ---
 (define fb-forge-serialize
   (fn ()
@@ -110,5 +131,6 @@
       {:review "sx-review" :baseline fb-baseline :done fb-done :start fb-start
        :agents (agentic/agents fb-sp)
        :workstream-cids fb-ws-cids
+       :writeback fb-writeback-cids
        :kv (map (fn (k) (list k ((get fb-db :kv-get) k))) ((get fb-db :kv-keys)))
        :streams (map (fn (s) (list s ((get fb-db :read) s))) ((get fb-db :streams)))})))

plans/sx-review/writeback.sxsrc

@@ -0,0 +1,11 @@
+; writeback.sxsrc — real-git commits recorded into the forge as agentic-sx commits.
+; APPENDED by scripts/sx-fix-writeback.sh; OVERRIDES fb-writeback-records defined
+; (empty) in forge-build.sxsrc, then replayed by (fb-do-writeback). Order is
+; chronological (oldest first) so each agent branch head advances correctly.
+;
+; Each record: {:agent "<forge-agent>" :kind "<agentic-kind>" :sha "<short-sha>"
+;               :branch "<git-branch>" :message "<subject>" :files "<space-sep paths>"}
+(define fb-writeback-records (list
+  {:agent "ws-W14" :kind "test" :sha "f09368e1" :branch "loops/sx-ws-w14" :message "W14: pin K18 expt-overflow float-promotion (test-only) + bootstrap gate briefing " :files "plans/agent-briefings/sx-gate-loop.md spec/tests/test-gate-pins.sx"}
+;;; END-RECORDS (scripts/sx-fix-writeback.sh inserts new records before this line)
+))