feat(mail): full Gmail+OnlyOffice sync, import, and brain indexing

- bin/mail/sync.go: async Go sync engine (8 workers, paginated Gmail via
  API + OnlyOffice IMAP); Gmail attachments key off body.attachmentId, not
  MIME partId; ICS sidecars Latin-1->UTF-8 normalized (TestICSToMarkdownNormalizesLatin1)
- bin/mail/import: message.json -> markdown; PDFs via pdftotext -layout
  fast path with docling subprocess fallback for the ~5% textless files
- bin/mail/index_mail: fresh-rebuild indexer (repo corpus + mail) avoiding
  ladybug WAL corruption on bulk-insert into indexed DBs; split from import
- bin/kb/index: keep FTS/VECTOR indexes across incremental runs (drop+recreate
  leaves stale backing tables killing the vector index)
- docs: README/PLAN/AGENTS cover the mail pipeline

Result: 17,835 messages -> 28,918 info leafs, FTS+HNSW healthy.

README.md

@@ -93,6 +93,15 @@ bin/kb/stats                                      # index health
 bin/kb/eval                                       # recall@5 gate
 ```
 
+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/import --from-raw var/mail                                     # JSON → markdown
+bin/mail/index_mail                                                     # rebuild brain incl. mail
+bin/kb/search "Mietwagen Nürnberg invoice"                              # now answers from mail
+```
+
 ## Storage
 
 - **LadybugDB** — single `var/kb.lbug`, Cypher property graph, HNSW + BM25