Plugin reload flow reworked with finer-grained status reporting

What

• reloadPlugins was replaced with a sessionPlugins object that exposes a reload() method (same behavior as before) plus a new retarget() method for repointing plugins.
• Reloading a plugin's hooks manifest now returns a result that distinguishes whether to retry or stop, instead of a generic pass/fail.
• The /config plugin-reload screen now reads each module's outcome (reloaded, refused, or held) to build its status message, rather than catching a generic error string.

Why This gives clearer feedback when Claude Code reloads plugin hooks: instead of a vague error, you can see specifically whether a module reloaded successfully, was refused, or is being held, making it easier to diagnose plugin problems.