Issue 56: Remove try/catch from skill module directory scanning

Replace the try/catch block in collect_paths() lambda with explicit
std::error_code-based error handling using recursive_directory_iterator
constructor and increment() methods that don't throw exceptions.

Error messages include the path that failed and the underlying error
message from the filesystem operation.

Added test_skill_unreadable_dir.sh to verify proper error handling when
a directory is unreadable.

src/skill.cpp

@@ -127,14 +127,24 @@ std::expected<std::vector<SkillMetadata>, std::string> scan_skills() {
         if (!fs::exists(path) || !fs::is_directory(path)) {
             return {};  // Silently skip non-existent directories
         }
-        try {
-            for (const auto& entry : fs::recursive_directory_iterator(path)) {
-                if (entry.is_regular_file() && entry.path().filename() == "SKILL.md") {
-                    discovered_paths.push_back(entry.path().string());
-                }
+
+        std::error_code ec;
+        auto it = fs::recursive_directory_iterator(path, ec);
+        if (ec) {
+            return std::unexpected("Could not scan directory " + path + ": " + ec.message());
+        }
+
+        auto end = fs::recursive_directory_iterator();
+
+        while (it != end) {
+            if (it->is_regular_file() && it->path().filename() == "SKILL.md") {
+                discovered_paths.push_back(it->path().string());
+            }
+
+            it.increment(ec);
+            if (ec) {
+                return std::unexpected("Could not scan directory " + path + ": " + ec.message());
             }
-        } catch (const fs::filesystem_error& e) {
-            return std::unexpected(std::string("Could not scan directory ") + path + ": " + e.what());
         }
         return {};
     };

tests/CMakeLists.txt

@@ -37,6 +37,7 @@ add_dr_test(test_editor)
 add_dr_test(test_tool_skill)
 add_dr_test(test_skill_filter)
 add_dr_test(test_skill_override)
+add_dr_test(test_skill_unreadable_dir)
 add_dr_test(test_auto_temp_file)
 
 # Add JSON parsing and error handling tests

tests/test_skill_unreadable_dir.sh

@@ -0,0 +1,41 @@
+#!/bin/bash
+# test_skill_unreadable_dir.sh - Verifies that unreadable directories produce proper error messages
+
+# Create a skill directory with proper content
+mkdir -p skills
+cat > skills/SKILL.md << 'SKILL'
+---
+name: test-skill
+---
+# Test Skill
+SKILL
+
+# Create a nested directory and make it unreadable
+mkdir -p skills/unreadable_subdir
+chmod 000 skills/unreadable_subdir
+
+# Run drharness with -D flag and capture stderr
+set +e
+OUTPUT=$("${DRHARNESS}" -D -F dummy.chat 2>&1)
+EXIT_CODE=$?
+set -e
+
+# Verify that it exits with non-zero code due to permission error
+if [ "$EXIT_CODE" -eq 0 ]; then
+    echo "FAIL: Expected non-zero exit code for unreadable directory, got 0"
+    chmod 755 skills/unreadable_subdir
+    exit 1
+fi
+
+# Verify the error message mentions the directory path
+if ! echo "$OUTPUT" | grep -q "Could not scan directory"; then
+    echo "FAIL: Expected 'Could not scan directory' in error output"
+    echo "Got: $OUTPUT"
+    chmod 755 skills/unreadable_subdir
+    exit 1
+fi
+
+# Clean up (restore permissions before removing)
+chmod 755 skills/unreadable_subdir
+
+echo "PASS: test_skill_unreadable_dir"