feat: first cut at dev brain

docs/USER_GUIDE.md

@@ -78,6 +78,55 @@ Tool calls and results appear in the transcript. Use their copy actions for the
 complete, untruncated text; large outputs can also be opened from their saved
 file.
 
+## Dev Brain
+
+Dev Brain is a project-backed wiki for durable engineering knowledge. It lets
+the coding agent compile architecture, behavior, decisions, invariants,
+workflows, and relationships once, then retrieve that verified context in later
+chats. Registered projects remain authoritative; Dev Brain is not a general
+memory store and never replaces checking changed source code.
+
+To set it up:
+
+1. Create or choose a dedicated Obsidian vault. The folder must already contain
+   **.obsidian**.
+2. Open **Preferences > Dev Brain**, choose the vault, enable Dev Brain, and
+   save.
+3. In a project chat, ask the agent to build or refresh Dev Brain. Tell it the
+   questions and areas that matter most so it can maintain **purpose.md** and
+   compile a small set of useful topic pages instead of summarizing every file.
+
+Useful requests include “Build Dev Brain for these projects,” “Refresh Dev
+Brain after my recent changes,” “What does Dev Brain know about the cache
+invariants?”, and “Check the wiki, then verify this against the source.” Search
+and reads use a disposable local SQLite full-text index. The index checks vault
+files and cited project evidence before use, so no separate indexing service or
+manual refresh button is needed.
+
+The vault remains ordinary Obsidian Markdown:
+
+- **purpose.md** records its scope, priorities, and recurring questions.
+- **schema.md** is the detailed contract the agent follows when maintaining it.
+- **index.md** catalogs every managed topic page.
+- **log.md** is an append-only history of material updates.
+- Topic pages live under **projects/**, **subsystems/**, **concepts/**,
+  **decisions/**, **invariants/**, and **workflows/** as needed.
+
+Every topic cites exact project files and a Git revision or content hash.
+**Verified** pages may support answers. **Stale** pages are navigation leads
+whose evidence changed, and **needs-review** pages contain visible uncertainty;
+the agent must recheck project sources before relying on either. Wiki updates
+are validated as a complete batch before publication, so a broken link,
+out-of-bounds path, invalid citation, or stale verified claim leaves the last
+valid wiki untouched.
+
+DS4Server may modify only the four contract files and topic pages explicitly
+marked as Dev Brain-managed. It does not modify Obsidian settings, attachments,
+trash, hidden files, or unrelated notes. You can inspect, edit, link, and
+version the managed Markdown normally; invalid manual edits are reported rather
+than silently trusted. Disabling Dev Brain removes its tools and instructions
+from the built-in coding agent without deleting the vault.
+
 ## Views and sidebar
 
 Use **View > Show Sidebar** (`⌘B`) to hide or restore the project sidebar. The