Feat v0.2.9 builtin extension retirement

Stop auto-seeding 6 chat-centric extensions on startup. Move them to
standard packages with js/ layout and "requires": ["chat"] metadata.
Delete SeedBuiltinPackages, seed tests, Dockerfile COPY, and
extensions/builtin/ directory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/store/package_iface.go

@@ -12,8 +12,7 @@ type PackageStore interface {
 
 	// Seed upserts a package at startup. Does NOT overwrite the enabled
 	// flag if the row already exists (admin toggle survives restarts).
-	// Called by the page engine for core surfaces and by
-	// SeedBuiltinPackages() for builtin extensions.
+	// Called by the page engine for core surfaces.
 	Seed(ctx context.Context, id, title, source string, manifest map[string]any) error
 
 	// List returns all registered packages, ordered by source then title.
@@ -39,7 +38,7 @@ type PackageStore interface {
 	// ── Extended lifecycle ───────────────────────────────
 
 	// Create inserts a new package with all fields. Used by the install
-	// handler and by SeedBuiltinPackages() for first-time seeding.
+	// handler for .pkg uploads.
 	Create(ctx context.Context, pkg *PackageRegistration) error
 
 	// Update modifies mutable fields on an existing package.