script/install: do not stop on a missing closing comment

A .gitignore or .dockerignore lacking the canonical last line no longer
stops the script: each line the canonical file has never held goes
below the fetched content. An .editorconfig lacking it is replaced,
unless it has such a line, which stops the script.

The stub curl now writes files with no execute bit, as a real download
does, so the tests exercise the mode the script sets.

Model: opus-5-5

test/update-policy.sh

@@ -45,7 +45,7 @@ not_found() {
 
 serve_file() {
     [ -f "$canonical/$1" ] || not_found
-    cp "$canonical/$1" "$output"
+    cat "$canonical/$1" > "$output"
 }
 
 case "$url" in
@@ -703,12 +703,37 @@ test_own_script_stops() {
 }
 
 test_gitignore_lacks_canonical_last_line() {
-    start_scenario ".gitignore lacking the canonical last line stops"
-    printf '*.log\n/myapp\n' > "$dir/repo/.gitignore"
+    start_scenario ".gitignore lacking the canonical last line keeps the repo's own line below the canonical content"
+    printf '*.log\n\n# A comment of our own.\n/myapp\n' > "$dir/repo/.gitignore"
     commit_and_push_fixture .gitignore .prettierrc
     run_update_policy
+    expect_success
+    expect_file .gitignore << 'EOF'
+*.log
+*.tmp
+
+# Own entries go below this comment.
+/myapp
+EOF
+    expect_new_commit_not_pushed
+}
+
+test_dockerignore_lacks_canonical_last_line() {
+    start_scenario ".dockerignore lacking the canonical last line drops a line an earlier canonical commit held"
+    echo "**/*.log" > "$dir/repo/.dockerignore"
+    expect_former_canonical_line_replaced .dockerignore ".git"
+    expect_same_as_canonical .dockerignore
+}
+
+test_editorconfig_lacks_canonical_last_line() {
+    start_scenario ".editorconfig lacking the canonical last line with a line of its own stops"
+    printf 'root = true\n[*.py]\nindent_size = 2\n' > "$dir/repo/.editorconfig"
+    commit_and_push_fixture .editorconfig .prettierrc
+    run_update_policy
     expect_failure
-    expect_output ".gitignore lacks this line"
+    expect_output ".editorconfig lacks this line"
+    expect_output "[*.py]"
+    expect_output "indent_size = 2"
     expect_nothing_written
 }
 
@@ -861,6 +886,8 @@ main() {
     test_ignored_file_stops
     test_own_script_stops
     test_gitignore_lacks_canonical_last_line
+    test_dockerignore_lacks_canonical_last_line
+    test_editorconfig_lacks_canonical_last_line
     test_fetched_golangci_without_deny_list
     test_own_golangci_setting_stops
     test_source_repo_stops