Implement SAND-WP-0005: extension SDK and ext.vm-packer

Add SandboxExtension base class, extension SDK docs, vm-packer attach mode
for build-machines VMs, profile.vm-haskell-build, SSH port support, tests,
and migration docs.

extensions/ext.vm-packer.yaml

@@ -0,0 +1,16 @@
+id: ext.vm-packer
+title: VM workspace (Packer lineage)
+description: >
+  Attach an isolated workspace on a pre-built VM (the-custodian build-machines
+  lineage). v0 supports attach mode via SSH alias or tunnel port; Packer build
+  orchestration is operator-driven and deferred.
+handler: sandboxer.extensions.vm_packer:VMPackerExtension
+capabilities:
+  isolation_levels: [microvm]
+  regions: []
+  persistence: true
+  pricing_model: self-hosted
+config:
+  workspace_base: /build
+  ssh_user: build
+  ready_timeout_s: 30