feat(rag): steering notes — tune how Brain answers, stored in Postgres and injected into every system prompt

README.md

@@ -85,6 +85,34 @@ uv run uvicorn app.main:app --reload
 - **Sources** (`/sources.html`) — the indexed document list; the *Path*
   column links each document to the viewer in a new tab.
 
+## Tuning your answers
+
+If an answer isn't quite right — too chatty, wrong assumption, missing
+context — **tune** Brain right there:
+
+1. Press **“Tune”** in the meta row under any completed answer (deflected
+   ones included).
+2. Type a short instruction (1–2000 chars), e.g. *“be more concise”* or
+   *“assume I'm on NixOS”*, and **Save**.
+
+The note is stored in Postgres (`steering_notes`) and read into the
+**system prompt of every subsequent chat turn** as a `<tuning>` section
+(numbered, oldest first, capped at `BOR_STEERING_MAX_CHARS` chars —
+default 8000, overflow marked `[…truncated…]`). With no stored notes the
+prompt is byte-identical to the un-tuned one, so tuning is opt-in per
+note.
+
+List or remove notes at any time from the **“Tuning”** button in the chat
+header (count badge, newest-first, per-note delete). The API is stateless
+JSON if you prefer curl:
+
+```bash
+curl -s localhost:8000/api/steering                                  # list (newest first)
+curl -s -X POST localhost:8000/api/steering \
+     -H 'Content-Type: application/json' -d '{"note": "be more concise"}'
+curl -s -X DELETE localhost:8000/api/steering/<note-id>              # remove
+```
+
 ## Updating the documents
 
 **This is the workflow you'll use most.** The knowledge base is refreshed by
@@ -270,6 +298,7 @@ served locally (no CDN), `BOR_ENVIRONMENT=production`.
 | `BOR_RRF_K` | `60` | RRF damping constant (`1/(k + rank)`) |
 | `BOR_IMPORT_EXTENSIONS` | `md,markdown,txt,yaml,yml,json,py` | csv of importable formats (may only narrow the A9 set) |
 | `BOR_MAX_CONTEXT_CHARS` | `24000` | cap on total document text sent to the LLM |
+| `BOR_STEERING_MAX_CHARS` | `8000` | char budget for the `<tuning>` (steering notes) prompt section |
 | `BOR_SUGGESTIONS` | built-in list | JSON list of onboarding chips |
 | `DEBUGPY` | `0` | `1` ⇒ attach-on-demand debugpy on `DEBUGPY_PORT` (default 5678) |
 | `BOR_LOG_LEVEL` | `INFO` | app log level |