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.

pyproject.toml

@@ -6,7 +6,9 @@ readme = "README.md"
 requires-python = ">=3.12"
 license = { text = "MIT" }
 dependencies = [
+    "docling>=2.119.0",
     "ladybug==0.19.1",
+    "markitdown[docx,epub,html,image-exif,pdf,pptx,xlsx,zip]>=0.1.7",
     "mistune==3.3.4",
     "model2vec==0.8.2",
     "numpy>=2.5.2",