feat(gmail-single-user): new single-mailbox Gmail integration

Gmail for one mailbox over OAuth 2.0 (no service account / delegation):
auth-link + exchange-code to obtain a refresh token, connectivity test,
message search/get, send/reply with attachments, attachment retrieval,
and get_incidents ingestion with an OCSF mapper. Refresh-token grant,
stdlib-only (no extra Python dependencies).

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

integrations/gmail-single-user/mappers/get_incidents.yaml

@@ -0,0 +1,15 @@
+name: "Gmail Single User Messages → OCSF"
+description: "Maps a Gmail message (get_incidents, results_path = result) to OCSF finding fields. Messages are flattened by the fetch script (subject/from/to/date extracted from headers)."
+field_mappings:
+  title: "subject"
+  severity: "2"
+  description: "snippet"
+ocsf:
+  - { source_path: "id",        ocsf_field: "finding_info.uid" }
+  - { source_path: "thread_id", ocsf_field: "finding_info.uid_alt" }
+  - { source_path: "subject",   ocsf_field: "finding_info.title" }
+  - { source_path: "snippet",   ocsf_field: "finding_info.desc" }
+  - { source_path: "date",      ocsf_field: "finding_info.created_time" }
+  - { source_path: "labels",    ocsf_field: "finding_info.types" }
+  - { source_path: "from",      ocsf_field: "actor.user.name" }
+  - { source_path: "to",        ocsf_field: "user.name" }