feat(mail): M365 Graph sync + bot ETL loop (OO+Gmail) (#38)

* feat(mail): add Microsoft 365 Graph sync source with delta state

Adds a GET-only Microsoft Graph source to bin/mail/sync (client-credentials,
delta query). Wires it into the CLI (--source m365), the worker pool and the
compose stack:

- m365.go: M365Client (token cache, delta pagination, message normalization,
  attachment download) + m365Source adapter (per-mailbox folders + delta link)
- sync.go: M365Credentials in SyncConfig; Committer interface so delta links
  only advance after a fully successful run (no skips on failure)
- cli.go: --source m365 with M365_/MS_ env passthrough
- Dockerfile: mail-build stage produces /mail-sync into the index image
- docker-entrypoint: mail-sync loop (sync -> import -> index, default 10s)
- compose.yaml: mail-sync service (index image, kb-var volume, secrets ro)

* 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.

bin/mail/sync.go

@@ -1,8 +1,9 @@
 //usr/bin/env go run "$0" "$@"; exit
-// bin/mail/sync.go - async download of OnlyOffice and Gmail mail to var/mail/.
+// bin/mail/sync.go - async download of OnlyOffice, Gmail and M365 mail to var/mail/.
 //
-//	./bin/mail/sync.go --source onlyoffice,gmail --limit 50 --workers 8
+//	./bin/mail/sync.go --source onlyoffice,gmail,m365 --limit 50 --workers 8
 //	./bin/mail/sync.go --source gmail --force
+//	./bin/mail/sync.go --source m365 --env .secrets/m365.env
 //	./bin/mail/sync.go --dry-run
 //
 // Writes raw message.json + attachments under var/mail/<folder>/<id>/; run