Implement ext.bwrap: local bubblewrap namespace extension (SAND-WP-0013)

Adds the first local, same-host, kernel-namespace-only sandbox
extension: no SSH hop, no container runtime. Extends IsolationSpec.level
with "process", implements BwrapExtension (provision/wait_ready/
teardown) spawning bwrap with unshared user/mount/pid/ipc/uts/net
namespaces, registers ext.bwrap + profile.bwrap-local, and extends
manager._handle_from_status to carry pid/workspace_dir. Verified with a
live bwrap smoke run in addition to the mocked test suite.

T04 (reachability vs. the SSH-based glas-harness consumer contract)
deliberately left open pending glas-harness's harness contract.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/sandboxer/models.py

@@ -41,7 +41,7 @@ class Consumer(BaseModel):
 
 
 class IsolationSpec(BaseModel):
-    level: Literal["container", "microvm", "policy"] = "container"
+    level: Literal["container", "microvm", "policy", "process"] = "container"
 
 
 class NetworkSpec(BaseModel):