#15618: fix(plugins): reject async plugin registration instead of silently ignoring

## Summary - When a plugin's `register()` returned a Promise, only a warn diagnostic was emitted - The plugin was still pushed to the registry as "loaded" despite incomplete registration - Any tools, hooks, or channels registered in the async portion were silently lost ## Fix - Mark async-registering plugins with `status: "error"` instead of marking them as loaded - Upgrade diagnostic from warn to error level - Use `continue` to skip the normal registration path ## Test plan - [x] Existing plugin loader tests pass <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change updates the plugin loader (`src/plugins/loader.ts`) to treat plugins whose `register()` returns a Promise as a hard error rather than a warning. In that case, the loader now marks the plugin record with `status: "error"`, attaches an error message, emits an `error`-level diagnostic, and `continue`s to skip the normal successful-registration path. This aligns the registry state (`registry.plugins`) with the system’s actual behavior: async work after `register()` cannot be awaited here, so any tools/hooks/channels registered later would otherwise be silently lost while the plugin still appeared "loaded". <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is narrowly scoped to the plugin registration path, preserves existing control flow for synchronous plugins, and improves correctness by preventing async-registering plugins from being reported as loaded. No additional behavior changes were found outside the intended error handling and diagnostic level update. - No files require special attention _{Last reviewed commit: e4df1d0} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->