chore: change slash command directory from .claude to .owlen

Changes slash command directory from `.claude/commands/` to
`.owlen/commands/` to reflect that owlen is its own tool while
maintaining compatibility with claude-code slash command syntax.

Updated locations:
- CLI main: command file path lookup
- Tests: slash_command_works and slash_command_file_refs

All 56 tests passing.

crates/app/cli/src/main.rs

@@ -185,8 +185,8 @@ async fn main() -> Result<()> {
                 // Check permission
                 match perms.check(Tool::SlashCommand, None) {
                     PermissionDecision::Allow => {
-                        // Look for command file in .claude/commands/
-                        let command_path = format!(".claude/commands/{}.md", command_name);
+                        // Look for command file in .owlen/commands/
+                        let command_path = format!(".owlen/commands/{}.md", command_name);
 
                         // Read the command file
                         let content = match tools_fs::read_file(&command_path) {