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.

.claude/rules/stack-and-commands.md

@@ -38,6 +38,7 @@ Sandbox CLI (v0):
 ```bash
 sandboxer create                              # canary self-deploy (profile.sandbox-canary)
 sandboxer create --profile profile.compose-e2e --input repo=/path/to/repo
+sandboxer create --profile profile.vm-haskell-build --input vm=haskell-build --input repo=/path
 sandboxer get <id>
 sandboxer list
 sandboxer destroy <id>