feat(mail): adapt M365 ETL loop for OO+Gmail bots.

Default compose mail-sync to onlyoffice,gmail every 300s with import on new
mail; gate full --rebuild behind MAIL_SYNC_INDEX. Fix mail-build stage order
and add bin/stack/start-mail-sync.

Dockerfile

@@ -6,6 +6,15 @@
 # API: Go + ladybug via Zig CGO (no CPython).
 # Index: Python write path (profile `index` until brain/add is v2).
 
+# --- mail-sync: standalone M365/OnlyOffice/Gmail puller (pure Go, no CGO) ---
+FROM golang:1.26-bookworm AS mail-build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY bin/mail ./bin/mail
+COPY internal ./internal
+RUN CGO_ENABLED=0 go build -o /mail-sync ./bin/mail/sync.go
+
 # --- Python sidecar (Ladybug write / rebuild) ---
 FROM python:3.12-slim AS index
 
@@ -38,14 +47,6 @@ HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
     CMD python -c "import model2vec, ladybug, mistune; print('ok')" || exit 1
 ENTRYPOINT ["/app/bin/docker-entrypoint"]
 
-# --- mail-sync: standalone M365/OnlyOffice/Gmail puller (pure Go, no CGO) ---
-FROM golang:1.26-bookworm AS mail-build
-WORKDIR /src
-COPY go.mod go.sum ./
-RUN go mod download
-COPY bin/mail ./bin/mail
-RUN CGO_ENABLED=0 go build -o /mail-sync ./bin/mail/sync.go
-
 # --- Go API: CGO with Zig, not gcc ---
 FROM golang:1.26-bookworm AS api-build
 WORKDIR /src