✨ feat: verificar dependências fzf e tmux ao iniciar

Adiciona módulo `deps` que, antes de qualquer operação, verifica se
fzf e tmux estão instalados no sistema. Caso faltem, detecta o
gerenciador de pacotes da distro (apt, pacman, dnf, yum, zypper,
emerge, xbps, apk), informa ao usuário e oferece instalar com o
comando adequado.

- `src/deps.rs`: Dep, PackageManager, BinaryChecker (trait injetável),
  check_missing(), detect_package_manager(), ensure_dependencies()
- `src/main.rs`: chama ensure_dependencies() antes do fluxo principal
- `tests/deps.rs`: 11 testes de integração com SystemBinaryChecker real
- `tests/docker/`: Dockerfile multi-stage + suite de 15 testes em
  container Ubuntu 24.04 simulando novo usuário (sem fzf/tmux)
- `.dockerignore`: exclui target/, .git/, .claude/ do contexto Docker

tests/docker/Dockerfile

@@ -0,0 +1,43 @@
+# syntax=docker/dockerfile:1
+
+# ---- Stage 1: Build (Rust stable on Debian slim) ----
+FROM rust:1-slim AS builder
+
+WORKDIR /src
+
+# Copy manifests first so cargo can resolve deps (layer cache friendly)
+COPY Cargo.toml Cargo.lock rust-toolchain.toml ./
+
+# Copy source and build release binary
+COPY src/ ./src/
+RUN cargo build --release --locked
+
+# ---- Stage 2: Test environment (fresh Ubuntu, no fzf/tmux) ----
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Install only what's needed to run the test suite itself
+# (git + sudo so Test 7 can install fzf/tmux via apt)
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        sudo \
+        git \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create an unprivileged user with passwordless sudo
+# (simulates a regular developer who can install packages)
+RUN useradd -m -s /bin/bash testuser \
+    && echo "testuser ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers
+
+# Install the tmuxido binary built in stage 1
+COPY --from=builder /src/target/release/tmuxido /usr/local/bin/tmuxido
+
+# Copy and register the test entrypoint
+COPY tests/docker/entrypoint.sh /usr/local/bin/entrypoint.sh
+RUN chmod +x /usr/local/bin/entrypoint.sh
+
+USER testuser
+WORKDIR /home/testuser
+
+ENTRYPOINT ["entrypoint.sh"]