The remaining writers created or rewrote brain files by path: - pipeline buildAndWrite: os.MkdirAll + os.WriteFile of every page. - BackfillRefs: os.WriteFile of each concept page; a page swapped for a symlink after the read was written through (the race test overwrote an in-brain target before this change). - watcher: the raw/processed and raw/failed archive copies (os.Create), the .processed/.failed markers and the log.md entry (os.OpenFile). - cmd/server: the claude-session archive. All now go through brainfs PutRegular/AppendRegular (new package-level PutFile/AppendFile for one-off writers: the watcher's call-site lint bans .Open calls in that file so raw/ reads stay on OpenRegular). Stacked on #189 (needs PutRegular/AppendRegular). Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>