fix: Dockerfile build mem-cli binary + CI builds image on PRs

- rust:1.81 → rust:1-bookworm (track latest stable)
- cargo build --release → cargo build --release -p mem-cli
- Docker build runs on PRs (verify before merge)
- Push only on main/workflow_dispatch

Dockerfile

@@ -1,15 +1,15 @@
 # Multi-stage build for Poimen Memory Service (Rust)
 
 # Stage 1: Builder
-FROM rust:1.81-bookworm as builder
+FROM rust:1-bookworm as builder
 
 WORKDIR /build
 
 # Copy source
 COPY . .
 
-# Build in release mode
-RUN cargo build --release
+# Build the mem binary
+RUN cargo build --release -p mem-cli
 
 # Stage 2: Runtime
 FROM debian:bookworm-slim