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.

bin/mail/sync.go

@@ -0,0 +1,25 @@
+//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 --source onlyoffice,gmail --limit 50 --workers 8
+//	./bin/mail/sync.go --source gmail --force
+//	./bin/mail/sync.go --dry-run
+//
+// Writes raw message.json + attachments under var/mail/<folder>/<id>/; run
+// bin/mail/import --from-raw afterwards to convert everything to markdown.
+//
+// Shebang trick: first line is a Go `//` comment; the real code lives in the
+// importable package (module path, never a relative import).
+// NOTE: never run `gofmt -w` on this file - it rewrites `//usr/bin/env` to
+// `// usr/...` and breaks the shebang.
+package main
+
+import (
+	"os"
+
+	"github.com/eSlider/2dph/bin/mail/sync"
+)
+
+func main() {
+	os.Exit(sync.Main(os.Args[1:]))
+}