Add Daytona sandbox defaults to fabro.toml and smoke workflow

Set Daytona as the project-level default sandbox so workflows that don't
specify their own sandbox config run on Daytona automatically. Add a
smoke workflow that verifies the sandbox toolchain (git, rustc, bun).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

fabro.toml

@@ -3,3 +3,35 @@ version = 1
 [fabro]
 root = "fabro/"
 retro = false
+
+[sandbox]
+provider = "daytona"
+
+[sandbox.daytona]
+auto_stop_interval = 30
+
+[sandbox.daytona.labels]
+repo = "fabro-sh/fabro"
+
+[sandbox.daytona.snapshot]
+name = "fabro-v2"
+cpu = 4
+memory = 8
+disk = 20
+dockerfile = """
+FROM ubuntu:24.04
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        curl git ca-certificates build-essential pkg-config libssl-dev unzip \
+    && rm -rf /var/lib/apt/lists/*
+
+# Rust
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+ENV PATH="/root/.cargo/bin:${PATH}"
+
+# Bun
+RUN curl -fsSL https://bun.sh/install | bash
+ENV PATH="/root/.bun/bin:${PATH}"
+
+WORKDIR /root
+"""

fabro/workflows/smoke/workflow.fabro

@@ -0,0 +1,11 @@
+digraph Smoke {
+    graph [goal="Verify the sandbox is working"]
+    rankdir=LR
+
+    start [shape=Mdiamond, label="Start"]
+    exit  [shape=Msquare, label="Exit"]
+
+    main [label="Smoke Test", prompt="Run these commands and report the results:\n1. echo 'Hello from Daytona'\n2. uname -a\n3. which git && git --version\n4. which rustc && rustc --version || echo 'no rustc'\n5. which bun && bun --version || echo 'no bun'\n\nReport what you found."]
+
+    start -> main -> exit
+}

fabro/workflows/smoke/workflow.toml

@@ -0,0 +1 @@
+version = 1