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>

extensions/ext.bwrap.yaml

@@ -0,0 +1,17 @@
+id: ext.bwrap
+title: Bubblewrap local namespaces
+description: >
+  Self-hosted extension using bubblewrap (bwrap) kernel namespaces on the
+  local host. No SSH hop, no container runtime, no remote placement — the
+  fastest path to an isolated workspace, and the first extension where
+  network.default: deny is real (a genuine namespace with no interface)
+  rather than declarative only.
+handler: sandboxer.extensions.bwrap:BwrapExtension
+capabilities:
+  isolation_levels: [process]
+  regions: []
+  persistence: false
+  pricing_model: self-hosted
+config:
+  base_dir: /tmp/sandboxer-bwrap
+  # ro_binds omitted — defaults to [/usr, /bin, /lib, /lib64, /etc/resolv.conf], filtered to what exists