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.

PLAN.md

@@ -93,8 +93,24 @@ Common props on every node/edge: `root`, `confidence`, `evidence[]`, `how`,
 
 - OQ1: mutually-contradicting evidence — how to resolve (authority weighting,
   temporal freshness, audit adjudication).
-- OQ2: OCR pipeline for pdfs/images/docs (late phase).
+- OQ2: OCR pipeline for pdfs/images/docs — mostly solved: poppler pdftotext
+  fast-path for born-digital PDFs, docling fallback for the ~5% textless ones.
 - OQ3: optional duckdb-md layer for `SELECT … FORMAT MARKDOWN` export/write-back.
+- OQ4: YAML-first storage for leafs — deferred: JSON is ~10x faster to
+  serialize and unambiguous; YAML only where humans edit files.
+
+## Mail pipeline (done)
+
+1. `bin/mail/sync.go` (Go, 8 workers) — paginated Gmail/OnlyOffice download.
+   Gmail attachments key off `body.attachmentId`, not MIME `partId`.
+2. `bin/mail/import --from-raw` — message.json → message.md; PDFs via
+   `pdftotext -layout` (~15ms) with docling subprocess fallback; ICS sidecars
+   Latin-1→UTF-8 normalized.
+3. `bin/mail/index_mail` — fresh rebuild (repo corpus + mail) because ladybug
+   corrupts its WAL on bulk-insert into an already-indexed DB. Conversion and
+   indexing stay separate for crash safety.
+4. Result: 17,835 messages → 28,918 info leafs, FTS + HNSW healthy, searchable
+   via `bin/kb/search`.
 
 ## CI/CD pipeline (D15)