Implement SAND-WP-0002 meta-framework foundation (T01–T09)

Add meta-framework spec, pydantic schemas, profile/extension YAML, extension
registry, ext.compose-ssh backend, SandboxManager with State Hub events, CLI
commands, integration docs, capability registry entry, and compose-e2e runbook.
Nine unit tests pass. T10 remote smoke test remains for operator.

tests/test_models.py

@@ -0,0 +1,61 @@
+"""Schema and loader tests."""
+
+from pathlib import Path
+
+import pytest
+import yaml
+
+from sandboxer.extensions.registry import load_all_extensions, load_extension
+from sandboxer.models import Consumer, Profile, SandboxCreateRequest
+from sandboxer.profiles.loader import load_profile
+
+
+def test_load_compose_e2e_profile() -> None:
+    profile = load_profile("profile.compose-e2e")
+    assert profile.extension == "ext.compose-ssh"
+    assert profile.placement.prefer == ["sandboxer01"]
+
+
+def test_load_compose_ssh_extension() -> None:
+    ext = load_extension("ext.compose-ssh")
+    assert ext.handler.endswith("ComposeSSHExtension")
+    assert "container" in ext.capabilities.isolation_levels
+
+
+def test_load_all_extensions_no_duplicates() -> None:
+    extensions = load_all_extensions()
+    assert "ext.compose-ssh" in extensions
+    assert len(extensions) == len(set(extensions))
+
+
+def test_profile_roundtrip(tmp_path: Path) -> None:
+    data = load_profile("profile.compose-e2e").model_dump()
+    path = tmp_path / "profile.test.yaml"
+    path.write_text(yaml.dump(data))
+    loaded = Profile.model_validate(yaml.safe_load(path.read_text()))
+    assert loaded.id == "profile.compose-e2e"
+
+
+def test_sandbox_create_request() -> None:
+    req = SandboxCreateRequest(
+        profile="profile.compose-e2e",
+        inputs={"repo": "/tmp/foo"},
+        consumer=Consumer(actor="adm", project="sand-boxer"),
+    )
+    assert req.inputs["repo"] == "/tmp/foo"
+
+
+def test_extension_missing_handler(tmp_path: Path) -> None:
+    bad = tmp_path / "ext.bad.yaml"
+    bad.write_text(
+        yaml.dump(
+            {
+                "id": "ext.bad",
+                "title": "Bad",
+                "handler": "",
+                "capabilities": {"isolation_levels": ["container"], "pricing_model": "self-hosted"},
+            }
+        )
+    )
+    with pytest.raises(ValueError, match="missing handler"):
+        load_extension("ext.bad", extensions_root=tmp_path)