agent permissions: drop hive-c0re's swarm-agent-permissions.json fallback, fail closed before the first read

Removes the HiveCopy hand-off: hive-c0re no longer writes each agent's
entry from swarm-agent-permissions.json into its runtime dir as
swarm-permissions.json, and the harness and subagent daemon no longer
read it. hive-c0re keeps the file as its own last read of the swarm for
the manage_root_agent mounts. Also removes permissions_import::pending(),
its only caller being the hand-off. No nix provisioned the copy, so no
nix option is removed and no tombstone is needed.

Rule after this change: until an agent's first successful read of its
key it holds no permissions (no execution, so no bash), and a subagent it
starts holds the same. After a read a set key wins, [] holds none and an
unset key is the default. A failed read keeps the last value, so an
unreachable queue at start keeps the agent closed until a read succeeds.
The harness logs once at warn that it starts closed.

Fixes #5315

docs/agent-lifecycle/persistence.md

@@ -430,9 +430,7 @@ holds the permissions the swarm grants this hive's agents, as
 `workers::swarm_permissions` last read them from the swarm's
 `agent-permissions` bucket (every 30 seconds): a copy, not the
 record, which `swarmctl agent permissions` sets. `set_nspawn_flags`
-reads it at every container start for `manage_root_agent`. Each agent gets
-its own entry from it as `swarm-permissions.json` in its runtime dir, which
-it holds until its own first read of its key succeeds.
+reads it at every container start for `manage_root_agent`.
 
 `/var/lib/hyperhive/swarm-agent-archetypes.json` holds the archetype each of
 this hive's archetype agents runs, as last read from the swarm's