Fix all shellcheck warnings for nix build gate

- SC2015: Wrap A && B || C patterns in brace groups for directive scope
- SC2064: Suppress for intentional early trap expansion (local vars)
- SC2164: Add || exit to cd commands in subshells
- SC2001: Suppress for sed URL injection (clearer than parameter expansion)
- SC1083: Handle {tree} git syntax (quote or suppress)
- SC1091: Suppress for runtime-resolved source paths
- SC2034: Remove unused exit codes (E_OK, E_CONFIG, E_AUTH)
- SC2116: Eliminate useless echo in mono_auth_url construction

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/sync.sh

@@ -18,6 +18,7 @@ forward_sync() {
   local subrepo_branch="$SYNC_BRANCH_SUBREPO"
   local work_dir
   work_dir=$(mktemp -d)
+  # shellcheck disable=SC2064  # Intentional early expansion — work_dir is local
   trap "rm -rf '$work_dir'" EXIT
 
   log "INFO" "=== Forward sync: mono/${mono_branch} → subrepo/${subrepo_branch} ==="
@@ -28,7 +29,7 @@ forward_sync() {
     --branch "$mono_branch" --single-branch \
     "${work_dir}/filtered" || die "Failed to clone through josh-proxy"
 
-  cd "${work_dir}/filtered"
+  cd "${work_dir}/filtered" || exit
   git config user.name "$BOT_NAME"
   git config user.email "$BOT_EMAIL"
 
@@ -56,7 +57,8 @@ forward_sync() {
 
   # 5. Compare trees — skip if identical
   local mono_tree subrepo_tree
-  mono_tree=$(git rev-parse HEAD^{tree})
+  mono_tree=$(git rev-parse 'HEAD^{tree}')
+  # shellcheck disable=SC1083  # {tree} is git syntax, not shell brace expansion
   subrepo_tree=$(git rev-parse "subrepo/${subrepo_branch}^{tree}" 2>/dev/null || echo "none")
 
   if [ "$mono_tree" = "$subrepo_tree" ]; then
@@ -110,8 +112,10 @@ ${BOT_TRAILER}: forward/${mono_branch}/$(date -u +%Y-%m-%dT%H:%M:%SZ)" >&2
     git push "$(subrepo_auth_url)" "${conflict_branch}"
 
     # Create PR on subrepo
-    local pr_body
-    pr_body="## Sync Conflict\n\nMonorepo \`${mono_branch}\` has changes that conflict with \`${subrepo_branch}\`.\n\n**Conflicted files:**\n$(echo "$conflicted" | sed 's/^/- /')\n\nPlease resolve and merge this PR to complete the sync."
+    local pr_body conflicted_list
+    # shellcheck disable=SC2001
+    conflicted_list=$(echo "$conflicted" | sed 's/^/- /')
+    pr_body="## Sync Conflict\n\nMonorepo \`${mono_branch}\` has changes that conflict with \`${subrepo_branch}\`.\n\n**Conflicted files:**\n${conflicted_list}\n\nPlease resolve and merge this PR to complete the sync."
 
     create_pr "${SUBREPO_API}" "${SUBREPO_TOKEN}" \
       "$subrepo_branch" "$conflict_branch" \
@@ -134,6 +138,7 @@ reverse_sync() {
   local subrepo_branch="$SYNC_BRANCH_SUBREPO"
   local work_dir
   work_dir=$(mktemp -d)
+  # shellcheck disable=SC2064  # Intentional early expansion — work_dir is local
   trap "rm -rf '$work_dir'" EXIT
 
   log "INFO" "=== Reverse sync: subrepo/${subrepo_branch} → mono/${mono_branch} ==="
@@ -143,7 +148,7 @@ reverse_sync() {
     --branch "$subrepo_branch" --single-branch \
     "${work_dir}/subrepo" || die "Failed to clone subrepo"
 
-  cd "${work_dir}/subrepo"
+  cd "${work_dir}/subrepo" || exit
   git config user.name "$BOT_NAME"
   git config user.email "$BOT_EMAIL"
 
@@ -205,13 +210,16 @@ initial_import() {
     '.[] | select(.name == $n) | .subfolder')
   local work_dir
   work_dir=$(mktemp -d)
+  # shellcheck disable=SC2064  # Intentional early expansion — work_dir is local
   trap "rm -rf '$work_dir'" EXIT
 
   log "INFO" "=== Initial import: subrepo/${subrepo_branch} → mono/${mono_branch}:${subfolder}/ ==="
 
   # 1. Clone monorepo (directly, not through josh — we need the real repo)
   local mono_auth_url
-  mono_auth_url=$(echo "https://${BOT_USER}:${GITEA_TOKEN}@$(echo "$MONOREPO_API" | sed 's|https://||; s|/api/v1/repos/|/|').git")
+  local api_host_path
+  api_host_path=$(echo "$MONOREPO_API" | sed 's|https://||; s|/api/v1/repos/|/|')
+  mono_auth_url="https://${BOT_USER}:${GITEA_TOKEN}@${api_host_path}.git"
 
   git clone "$mono_auth_url" \
     --branch "$mono_branch" --single-branch \
@@ -227,7 +235,7 @@ initial_import() {
   log "INFO" "Subrepo has ${file_count} files"
 
   # 3. Copy subrepo content into monorepo subfolder
-  cd "${work_dir}/monorepo"
+  cd "${work_dir}/monorepo" || exit
   git config user.name "$BOT_NAME"
   git config user.email "$BOT_EMAIL"
 
@@ -277,6 +285,7 @@ subrepo_reset() {
   local subrepo_branch="$SYNC_BRANCH_SUBREPO"
   local work_dir
   work_dir=$(mktemp -d)
+  # shellcheck disable=SC2064  # Intentional early expansion — work_dir is local
   trap "rm -rf '$work_dir'" EXIT
 
   log "INFO" "=== Subrepo reset: josh-filtered mono/${mono_branch} → subrepo/${subrepo_branch} ==="
@@ -287,7 +296,7 @@ subrepo_reset() {
     --branch "$mono_branch" --single-branch \
     "${work_dir}/filtered" || die "Failed to clone through josh-proxy"
 
-  cd "${work_dir}/filtered"
+  cd "${work_dir}/filtered" || exit
 
   local head_sha
   head_sha=$(git rev-parse HEAD)