Session 5: Dockerfile, config example, README rewrite

Multi-stage alpine Dockerfile builds with -tags goolm so libolm isn't
needed at runtime. Annotated config.example.yaml documents every field
and shows ${ENV_VAR} usage for secrets. README is rewritten for the Go
bot — Python-era web-portal docs are gone.

SESSION_PLAN.md

@@ -42,7 +42,7 @@ Rewriting Bellhop from a Python FastAPI web portal into a Go-based Matrix comman
   - "Service not configured" reply if the corresponding *arr block is absent
   - `main.go`: load config, init matrix, wire handler, SIGINT/SIGTERM shutdown
 
-- [ ] **Session 5 — Ship polish**
+- [x] **Session 5 — Ship polish**
   - Multi-stage Dockerfile (`golang:1.25-alpine` → `alpine:3.21`, build with `-tags goolm`)
   - `config.example.yaml`
   - Rewrite `README.md`: command UX, install, config reference, Docker