feat(db): add actor_client_id audit column + github_linked event kind

Adds nullable actor_client_id text column to report_events and
report_comments for MCP OAuth machine-actor attribution. Extends
the report_events kind enum with github_linked alongside github_unlinked.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Ripwords

apps/dashboard/server/db/migrations/0017_cute_toxin.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "report_comments" ADD COLUMN "actor_client_id" text;--> statement-breakpoint
+ALTER TABLE "report_events" ADD COLUMN "actor_client_id" text;