feat(box): add BoxProfile with resource limits and improved output truncation

- Implement head+tail output truncation (60/40 split) so LLM sees both
    beginning and final results; add streaming byte-limited reads in backend
    to prevent unbounded memory usage (_MAX_RAW_OUTPUT_BYTES = 1MB)
  - Define BoxProfile model with locked fields and max_timeout_sec clamping
  - Add four built-in profiles: default, offline_readonly, network_basic,
    network_extended with differentiated resource and security constraints
  - Add resource limit fields to BoxSpec (cpus, memory_mb, pids_limit,
    read_only_rootfs) and pass corresponding container CLI flags
    (--cpus, --memory, --pids-limit, --read-only, --tmpfs)
  - Profile loaded from config (box.profile), applied in service layer
    before BoxSpec validation; locked fields cannot be overridden by
    tool-call parameters

src/langbot/templates/config.yaml

@@ -88,6 +88,7 @@ monitoring:
         # Cleanup check interval in hours
         check_interval_hours: 1
 box:
+    profile: 'default'
     default_host_workspace: './data/box-workspaces/default'
     allowed_host_mount_roots:
         - './data/box-workspaces'