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.

tests/test_extension_base.py

@@ -0,0 +1,16 @@
+"""Extension SDK base class tests."""
+
+from sandboxer.extensions.base import SandboxExtension
+from sandboxer.extensions.compose_ssh import ComposeSSHExtension
+from sandboxer.extensions.vm_packer import VMPackerExtension
+
+
+def test_reference_extensions_subclass_base() -> None:
+    assert issubclass(ComposeSSHExtension, SandboxExtension)
+    assert issubclass(VMPackerExtension, SandboxExtension)
+
+
+def test_new_sandbox_id_from_inputs() -> None:
+    assert SandboxExtension.new_sandbox_id({"sandbox_id": "fixed123"}) == "fixed123"
+    generated = SandboxExtension.new_sandbox_id({})
+    assert len(generated) == 8