feat: migrate CI to Codeberg Actions and add self-hosted runner config

- Added .forgejo/workflows/ci.yml for thin CI orchestration.
- Configured Dockerized Codeberg runner with Nix support in codeberg-runner/.
- Added systemd service for persistent runner execution.
- Added GEMINI.md for project CI/Nix conventions.
- Added Taskfile task for Linux release builds.

codeberg-runner/Dockerfile

@@ -0,0 +1,18 @@
+# Dockerfile for a Codeberg Runner with Nix installed
+FROM gitea/act_runner:latest
+
+# Install Nix requirements and basic tools
+RUN apt-get update && apt-get install -y curl xz-utils sudo && rm -rf /var/lib/apt/lists/*
+
+# Install Nix in single-user mode (suitable for container)
+# We use the Determinate Systems installer for reliability
+RUN curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install linux \
+    --init none \
+    --no-confirm
+
+# Add Nix to PATH
+ENV PATH="/nix/var/nix/profiles/default/bin:${PATH}"
+ENV NIX_PATH="nixpkgs=channel:nixos-unstable"
+
+# Ensure the runner user can use Nix
+RUN echo "experimental-features = nix-command flakes" >> /etc/nix/nix.conf

codeberg-runner/codeberg-runner.service

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+[Unit]
+Description=Codeberg CI Runner (Docker Compose)
+Requires=docker.service
+After=docker.service network-online.target
+
+[Service]
+Type=simple
+WorkingDirectory=/home/picoclaw/projects/sharedinbox3/codeberg-runner
+ExecStartPre=-/usr/bin/docker compose down
+ExecStart=/usr/bin/docker compose up --build
+ExecStop=/usr/bin/docker compose down
+Restart=always
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target

codeberg-runner/docker-compose.yml

@@ -0,0 +1,19 @@
+services:
+  runner:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    restart: always
+    env_file:
+      - ../.env
+    environment:
+      - GITEA_INSTANCE_URL=${CODEBERG_INSTANCE_URL:-https://codeberg.org}
+      - GITEA_RUNNER_REGISTRATION_TOKEN=${CODEBERG_CI_RUNNER_TOKEN}
+      - GITEA_RUNNER_NAME=${CODEBERG_RUNNER_NAME:-laptop-runner}
+      - GITEA_RUNNER_LABELS=${CODEBERG_RUNNER_LABELS:-self-hosted,linux,nix}
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+      - ./runner-data:/data
+    # Use host network if you want to access local services easily, 
+    # but for most cases the default bridge is fine.
+    # network_mode: host