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.

README.md

@@ -119,6 +119,8 @@ Mail is a first-class corpus (retrievable through the same search):
 
 ```bash
 bin/mail/sync.go --source onlyoffice,gmail --workers 8 --out var/mail  # raw sync (Go)
+bin/mail/sync.go --source m365 --env ~/.config/brain/mail.env          # Microsoft 365 Graph
+bin/stack/start-mail-sync                                              # compose ETL (300s; no auto-rebuild)
 bin/mail/import.go --from-raw var/mail                                  # JSON → markdown
 bin/brain/add.go --text T --root facts --source "a.md x b.md"
 bin/brain/index.go --rebuild --with-facts --with-chats                  # facts extract + chats md