feat: brain/index.go shebang; mail import is not a brain write (D14).

Commands live at bin/brain/{index,get,stats,eval,watch}.go and
bin/mail/import.go, bin/markdown/import.go, bin/postgres/query.go.
Python remains the Ladybug write worker. index_mail is a deprecation
shim that rebuilds via --with-mail.

skills/diataxis-docs/SKILL.md

@@ -30,7 +30,7 @@ related:
 ---
 ```
 
-`bin/kb/index` reads this. `type` becomes a searchable column. `related:` is
+`bin/brain/index.go` reads this. `type` becomes a searchable column. `related:` is
 frontmatter for humans; graph hops from it are not implemented yet.
 
 ```bash

skills/kb-search/SKILL.md

@@ -26,9 +26,9 @@ second independent source when local roots cannot confirm. An answer is
 bin/brain/search.go "Matrix federation"                # pointers + snippets, YAML
 bin/brain/search.go "onlyoffice postgres" --root facts # restrict to confirmed
 bin/brain/search.go "where is cs-lexicon" --json | yq '.[].ref'
-bin/kb/get <id> --body                                 # full chunk only when needed
-bin/kb/stats                                           # index health
-bin/kb/eval                                            # recall@5 >= 0.95 gate
+bin/brain/get.go <id> --body                           # full chunk only when needed
+bin/brain/stats.go                                     # index health
+bin/brain/eval.go                                      # recall@5 >= 0.95 gate
 ```
 
 `bin/kb/search` is a deprecated wrapper. `--hop` errors (File/FROM_FILE edges
@@ -39,7 +39,7 @@ are not wired yet); do not treat it as a graph walk.
 - Search before you read. Never grep a repo for a concept the graph covers.
 - `--root facts` returns only confirmed evidence-linked answers. Default shows
   facts first, then info leafs clearly marked `(not confirmed)`.
-- If recall looks wrong, run `bin/kb/eval`; it gates control questions and
+- If recall looks wrong, run `bin/brain/eval.go`; it gates control questions and
   should stay at or above 95% recall@5.
 - Escalate to `web-search` (the `web-search` skill) as the independent second
   source when both local roots cannot confirm; never report an unconfirmed