Initial: branch-oriented eval framework

scripts/start-eval.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ $# -ne 1 ]]; then
+    echo "Usage: $0 <eval-name>"
+    echo "Example: $0 opencode-glm47"
+    exit 1
+fi
+
+EVAL_NAME="eval/$1"
+EVAL_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+
+# Verify we're on main
+CURRENT_BRANCH=$(git branch --show-current)
+if [[ "${CURRENT_BRANCH}" != "main" ]]; then
+    echo "Error: Must be on 'main' branch to start an evaluation."
+    echo "Current branch: ${CURRENT_BRANCH}"
+    exit 1
+fi
+
+# Check if eval branch already exists
+if git show-ref --verify --quiet refs/heads/"${EVAL_NAME}"; then
+    echo "Error: Evaluation branch '${EVAL_NAME}' already exists."
+    echo "Use a different name or delete the existing branch first."
+    exit 1
+fi
+
+echo "Creating evaluation branch: ${EVAL_NAME}"
+
+# Create orphan branch
+git switch --orphan "${EVAL_NAME}"
+
+# Copy only flake files from main
+git checkout main -- flake.nix flake.lock .envrc SPEC.md
+
+# Initial commit
+git add .
+git commit -m "Initial: setup evaluation environment"
+
+# Set up direnv
+direnv allow
+
+echo ""
+echo "Evaluation environment ready!"
+echo "Working on branch: ${EVAL_NAME}"
+echo ""
+echo "Run 'git checkout main' when you're done to return to main."