feat(firecracker): group-owned TAP pool for multi-user hosts

Add a `group` option to the netpool NixOS module + BOT_BOTTLE_FC_GROUP to
the shell script: when set, the pool's TAP devices are owned by a group
instead of a single user, so any group member can open them (the kernel
lets a TAP's owning-group members attach). This lets an interactive user
and, say, a CI-runner user share one pool. `owner`/`group` are mutually
exclusive (asserted). Single-user `owner` remains the default.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

tests/unit/test_firecracker_backend.py

@@ -69,6 +69,8 @@ class TestNetpoolRenderers(unittest.TestCase):
         self.assertIn("bot-bottle-firecracker-netpool", mod)  # the oneshot
         self.assertIn(netpool.NFT_TABLE, mod)
         self.assertIn('iifname != "${cfg.ifacePrefix}*" return', mod)
+        # Supports group-owned TAPs (shared pool for a multi-user host).
+        self.assertIn("group ${cfg.group}", mod)
 
     def test_shell_setup_reflects_overrides(self):
         with patch.dict(os.environ, {"BOT_BOTTLE_FC_POOL_SIZE": "3"}):