refactor(use-skills): simplify execution flow

- Add set -euo pipefail for safer defaults
- Remove conditional directory logic - always create both
- Remove undocumented SKILLS_CLAUDE/SKILLS_OPENCODE env vars
- Add fail-fast error handling throughout
- Capture nix build stderr for better error messages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

bin/use-skills.sh

@@ -3,17 +3,16 @@
 # Source this from your .envrc or copy the functions
 #
 # Usage in .envrc:
-#   source /path/to/skills/bin/use-skills.sh
+#   source ~/proj/skills/bin/use-skills.sh
 #   use_skills worklog web-search
 #
-# Or without sourcing (copy-paste into .envrc):
-#   SKILLS_REPO="git+ssh://git@forgejo.example/dan/skills.git"
-#   use_skill() { ... }
-#   use_skill worklog
+# Or with manifest file:
+#   source ~/proj/skills/bin/use-skills.sh
+#   load_skills_from_manifest
+
+set -euo pipefail
 
 # Default repo - uses local git, override with SKILLS_REPO for remote
-# Local: git+file:///home/dan/proj/skills (default, works offline)
-# Network: git+http://192.168.1.108:3000/dan/skills.git
 SKILLS_REPO="${SKILLS_REPO:-git+file://$HOME/proj/skills}"
 
 # Install a single skill via nix build + symlink
@@ -21,48 +20,34 @@ use_skill() {
   local skill="$1"
   local out
 
-  out=$(nix build --print-out-paths --no-link "${SKILLS_REPO}#${skill}" 2>/dev/null)
-  if [[ -z "$out" ]]; then
+  out=$(nix build --print-out-paths --no-link "${SKILLS_REPO}#${skill}" 2>&1) || {
     echo "use_skill: failed to build ${skill}" >&2
+    echo "$out" >&2
     return 1
-  fi
+  }
 
-  # Claude Code
-  if [[ -d .claude ]] || [[ -n "${SKILLS_CLAUDE:-}" ]]; then
-    mkdir -p .claude/skills
-    ln -sfn "$out" ".claude/skills/${skill}"
-  fi
-
-  # OpenCode
-  if [[ -d .opencode ]] || [[ -n "${SKILLS_OPENCODE:-}" ]]; then
-    mkdir -p .opencode/skills
-    ln -sfn "$out" ".opencode/skills/${skill}"
-  fi
-
-  echo "use_skill: ${skill} -> ${out}"
+  mkdir -p .claude/skills .opencode/skills
+  ln -sfn "$out" ".claude/skills/${skill}"
+  ln -sfn "$out" ".opencode/skills/${skill}"
+  echo "use_skill: ${skill}"
 }
 
 # Install multiple skills
 use_skills() {
-  # Ensure at least one target exists
-  mkdir -p .claude/skills .opencode/skills
-
   for skill in "$@"; do
-    use_skill "$skill"
+    use_skill "$skill" || return 1
   done
 }
 
 # Load skills from .skills manifest file
 load_skills_from_manifest() {
   [[ ! -f .skills ]] && return 0
-  mkdir -p .claude/skills .opencode/skills
-
   while IFS= read -r skill || [[ -n "$skill" ]]; do
     # Skip empty lines and comments
     [[ -z "$skill" || "$skill" =~ ^[[:space:]]*# ]] && continue
     # Strip inline comments and whitespace
     skill="${skill%%#*}"
     skill="${skill// /}"
-    [[ -n "$skill" ]] && use_skill "$skill"
+    [[ -n "$skill" ]] && { use_skill "$skill" || return 1; }
   done < .skills
 }