Add local reachability descriptor for same-host extensions (SAND-WP-0013-T04)

Reachability gains pid/workspace_dir, populated by enrich_reachability
whenever a backend's wait_ready() returns a pid: endpoint (currently
just ext.bwrap). build_reachability_report() now also returns a
local_exec_hint (nsenter into the pid's namespaces) alongside the
existing ssh_one_liner, so glas-harness can branch on which is
populated instead of assuming every sandbox is SSH-reachable.

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

docs/integrations/glas-harness.md

@@ -22,6 +22,23 @@ sandboxer create \
 | `reachability.remote_dir` | sand-boxer | Workspace root on remote host |
 | `state` | sand-boxer | Lifecycle state (`ready`, etc.) |
 
+## Two reachability modes
+
+Not every sandbox has an SSH hop. `ext.compose-ssh` / `ext.vm-packer`
+always run remote and populate `reachability.ssh` + `reachability.
+remote_dir`; glas-harness execs tools over that SSH channel. `ext.bwrap`
+(SAND-WP-0013) runs same-host and never populates `reachability.ssh` —
+instead it populates `reachability.pid` (the placeholder process holding
+the bwrap namespaces) and `reachability.workspace_dir`. glas-harness
+execs tools by entering that pid's namespaces directly
+(`nsenter --target <pid> --mount --pid --net --uts --ipc -- <cmd>`, no
+tunnel needed) rather than opening an SSH channel. `reachability/enrich.
+build_reachability_report()` returns both an `ssh_one_liner` and a
+`local_exec_hint`; exactly one is non-null depending on which mode the
+resolved extension uses. Consumers should branch on which field is
+populated, not on profile id, since routing can fall back between
+extensions.
+
 ## Ownership
 
 | Concern | Owner |