fix(internal#248): correct PluginResolver.Resolve return type to SourceResolver

The interface declared Resolve(Source) (PluginResolver, error) but
*Registry.Resolve in source.go:133 returns SourceResolver. Compile-time
assertion `var _ PluginResolver = (*Registry)(nil)` at drift_sweeper.go:82
failed → broke main.go:341 router.Setup and main.go:350 StartPluginDriftSweeper.

Doc comment at drift_sweeper.go:68-69 already stated the intended shape
(returns the production SourceResolver from source.go, not another
PluginResolver) — this aligns the type with the documented contract.

PluginsHandler.WithSourceResolver is unaffected: router.go:545-565
intentionally `_ = pluginResolver`s it out per existing comment.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/plugins/drift_sweeper.go

@@ -61,15 +61,26 @@ const DriftSweepInterval = 1 * time.Hour
 // that handles Gitea instances on high-latency links.
 const ResolveRefDeadline = 60 * time.Second
 
-// PluginResolver resolves plugin sources to installable directories.
-// Satisfied by *Registry (which wraps GithubResolver + LocalResolver).
+// PluginResolver is the registry-level abstraction the sweeper consumes:
+// pick a per-scheme SourceResolver for a parsed Source, and enumerate the
+// registered schemes so we can strip the prefix from a stored source_raw.
+//
+// Resolve returns the production SourceResolver from source.go (NOT another
+// PluginResolver) — that's the actual shape of *Registry.Resolve, and the
+// sweeper only needs the per-scheme resolver's identity, not its Fetch.
+//
 // Named PluginResolver (not SourceResolver) to avoid redeclaring the
-// SourceResolver interface defined in source.go (core#228 fix).
+// per-scheme SourceResolver interface defined in source.go (core#228 fix).
+// Satisfied by *Registry from source.go via Resolve + Schemes.
 type PluginResolver interface {
-	Resolve(source Source) (PluginResolver, error)
+	Resolve(source Source) (SourceResolver, error)
 	Schemes() []string
 }
 
+// Compile-time assertion: *Registry satisfies PluginResolver. Catches any
+// future drift in Registry.Resolve / Schemes signatures at build time.
+var _ PluginResolver = (*Registry)(nil)
+
 // StartPluginDriftSweeper runs the drift-detection loop until ctx is cancelled.
 // Pass a nil resolver to disable the sweeper (useful for harnesses or CP/SaaS
 // mode where git operations are unavailable).