Better diagnostics for plugin file-access failures

What

• Plugin loading now produces clearer diagnostics when something goes wrong reading a plugin's component folders: checking each default folder (commands/agents/skills/output-styles/themes/workflows) now logs an error and records a diagnostic entry when the failure is something other than "not found", instead of silently treating the folder as absent.
• When a plugin or marketplace manifest path can't be found and there's an associated system error number (errno), the error message now includes that errno and suggests likely causes, such as a self-referential symlink, a broken network mount, or a permissions problem, and tells the user to run /reload-plugins.

Why

These changes make it much easier to diagnose why a plugin failed to load, by surfacing the actual underlying filesystem error instead of just treating any failure as a missing file.