Add async RegistryClient implementing the Package Registry Standard
v1.0.0 HTTP API endpoints (§8) using httpx.AsyncClient.

- GET /packages/{id} — retrieve package content by PackageId
- GET /{type}/{ns}/{name}?version= — resolve references with
  version alias resolution (latest, vx, vX.Y.x, vX.x per §4.2)
- GET /browse — discover published packages with type/namespace filters
- GET /.well-known/cleverthis-packages — registry metadata discovery
- Maps all 8 standard error types (§13.2) to typed exceptions
- Supports anonymous reads (§9.1) and optional API key auth (§9.2)
- Async context manager support (__aenter__/__aexit__)
- 32 Behave BDD scenarios, 7 Robot Framework integration tests,
  ASV benchmarks, 100% registry module coverage

ISSUES CLOSED: #24

@@ -0,0 +139,4 @@
+
+  Scenario: Unknown error status falls back to RegistryError
+    When I create a RegistryClient pointed at "https://registry.example.com"
+    When the mock server returns status 418 with non-JSON body "plain text error"

@@ -0,0 +131,4 @@
+                future = asyncio.run_coroutine_threadsafe(coro_fn(), loop)
+                future.result(timeout=10)
+            else:
+                loop.run_until_complete(coro_fn())

@@ -0,0 +63,4 @@
+                    "packages": [],
+                    "count": 0,
+                },
+            )

@@ -0,0 +33,4 @@
+    "skill": "pkg_skl_",
+    "mcp": "pkg_mcp_",
+    "lsp": "pkg_lsp_",
+}

@@ -0,0 +68,4 @@
+    if status_code == 403:
+        return AccessDeniedError(message)
+    if status_code == 404:
+        return PackageNotFoundError(message)