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>

src/sandboxer/models.py

@@ -156,6 +156,11 @@ class Reachability(BaseModel):
     tunnel: str | None = None
     tunnel_via: str | None = None
     identity: str | None = None
+    # Local (no-SSH-hop) descriptor — populated for same-host extensions
+    # like ext.bwrap. A consumer execs into the sandbox directly
+    # (e.g. `nsenter --target <pid> ...`) rather than over SSH.
+    pid: str | None = None
+    workspace_dir: str | None = None
 
 
 class SandboxStatus(BaseModel):

src/sandboxer/reachability/enrich.py

@@ -19,6 +19,11 @@ def enrich_reachability(
     enriched = dict(reach)
     spec = profile.reachability
 
+    if handle.get("workspace_dir"):
+        enriched.setdefault("workspace_dir", handle["workspace_dir"])
+    if handle.get("pid") and enriched.get("endpoint", "").startswith("pid:"):
+        enriched.setdefault("pid", handle["pid"])
+
     if spec.tunnel:
         enriched.setdefault("tunnel_via", spec.tunnel)
     if spec.identity:
@@ -50,6 +55,20 @@ def ssh_one_liner(reach: Reachability) -> str | None:
     return None
 
 
+def local_exec_hint(reach: Reachability) -> str | None:
+    """No-SSH-hop exec hint for same-host extensions (e.g. ext.bwrap).
+
+    Consumers exec directly into the sandbox's namespaces via the
+    placeholder process's pid, rather than opening an SSH channel.
+    """
+    if reach.pid and reach.workspace_dir:
+        return (
+            f"nsenter --target {reach.pid} --mount --pid --net --uts --ipc "
+            f"-- sh -c 'cd {reach.workspace_dir} && exec $SHELL'"
+        )
+    return None
+
+
 def build_reachability_report(status: SandboxStatus) -> dict[str, Any]:
     """Consumer-facing reachability report with ops-bridge pointer."""
     reach = status.reachability
@@ -65,4 +84,5 @@ def build_reachability_report(status: SandboxStatus) -> dict[str, Any]:
     }
     if reach:
         payload["ssh_one_liner"] = ssh_one_liner(reach)
+        payload["local_exec_hint"] = local_exec_hint(reach)
     return payload