fix: change appname to neoirc and default DB to /var/lib/neoirc/state.db

- Change Appname from "chat" to "neoirc" in cmd/chatd/main.go
- Change default DBURL from file:./data.db to file:///var/lib/neoirc/state.db
  in both internal/config/config.go and internal/db/db.go fallback
- Create /var/lib/neoirc/ directory in Dockerfile with proper ownership
- Update README.md to reflect new defaults (DBURL, docker run example,
  SQLite backup/location docs)
- Remove stale data.db reference from Makefile clean target

The DB path remains configurable via DBURL environment variable.

Closes #44

Dockerfile

@@ -33,7 +33,9 @@ RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /chat-cli ./cmd/chat-cl
 # alpine:3.21, 2026-02-26
 FROM alpine@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
 RUN apk add --no-cache ca-certificates \
-    && addgroup -S chat && adduser -S chat -G chat
+    && addgroup -S chat && adduser -S chat -G chat \
+    && mkdir -p /var/lib/neoirc \
+    && chown chat:chat /var/lib/neoirc
 COPY --from=builder /chatd /usr/local/bin/chatd
 
 USER chat