refactor(plugins): delete dead plugin manager code

[datlechin]

Why

Cleanup sweep, S1: plugin manager code with no caller or no reader, and a fake install progress.

Change

• Delete what nothing calls or reads: validateDependencies, validateConnectionFields, validateDialectDescriptor, loadPendingPluginsAsync, outdatedReconcileReason, existingPluginLoaded, performRegistryInstall, the second markStaged, the permission API on PluginNotificationService, the category and tagline fallback table, and the unreachable re-validate and isOutdated paths.
• Plugin downloads report the task's own progress instead of a fixed 0, 0.5, 1. With no Content-Length the bar stays indeterminate.
• validateBundleVersions replaces validateStagedABI before atomicReplace; the checksum streams the file; no manual .bak removal.
• Delete the PluginKit history comment. The two version constants stay.

Verified

PluginInstallerHelpersTests: the validateStagedABI cases now run against validateBundleVersions, plus two download progress cases.

Not in this PR

• PluginActionResult stays: InstalledPluginsView still reads it.