Merge branch 'feat/shared-module'

Shared Go module extraction (spec 0a) and the Org to Instance rename (spec 0b).

- shared/ module holds the documents and provisioning rules the control plane
  and sitesvc both write; sitesvc's hand-copied duplicates are deleted
- Org becomes Instance everywhere, including the org_id field on all 17
  tenant-scoped collections, via migration 0004_org_to_instance
- cmd/rename-rollback reverses the migration
- Go images now build from the repo root so the replace directive resolves

Migration verified against a seeded legacy fixture: lossless, tenant-isolated,
idempotent, resumable and reversible. NOT yet rehearsed against a production
snapshot, which plan 0b requires before deploying.

internal/inventory/collect_other.go

@@ -1,5 +1,9 @@
+//go:build !linux
 
-
+// Inventory collection is Linux-only. This no-op stands in everywhere else.
+//
+// The build constraint above is load-bearing: "_other" is not a GOOS suffix, so
+// without it this file compiles on Linux too and collides with collect_linux.go.
 package inventory
 
 import "github.com/mrhid6/vantage/agent/internal/grpc/pb"