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

tests/unit_tests/box/test_backend_clip.py

@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+import pytest
+
+from langbot.pkg.box.backend import CLISandboxBackend, _MAX_RAW_OUTPUT_BYTES
+
+
+class TestClipBytes:
+    def test_within_limit_unchanged(self):
+        data = b'hello world'
+        result = CLISandboxBackend._clip_bytes(data, limit=1024)
+        assert result == 'hello world'
+
+    def test_exceeding_limit_clips_and_appends_notice(self):
+        data = b'A' * 200
+        result = CLISandboxBackend._clip_bytes(data, limit=100)
+        assert result.startswith('A' * 100)
+        assert 'raw output clipped at 100 bytes' in result
+        assert '100 bytes discarded' in result
+
+    def test_exact_limit_not_clipped(self):
+        data = b'B' * 100
+        result = CLISandboxBackend._clip_bytes(data, limit=100)
+        assert result == 'B' * 100
+        assert 'clipped' not in result
+
+    def test_default_limit_is_module_constant(self):
+        data = b'x' * 10
+        result = CLISandboxBackend._clip_bytes(data)
+        assert result == 'x' * 10
+        assert _MAX_RAW_OUTPUT_BYTES == 1_048_576
+
+    def test_invalid_utf8_replaced(self):
+        data = b'ok\xff\xfetail'
+        result = CLISandboxBackend._clip_bytes(data, limit=1024)
+        assert 'ok' in result
+        assert 'tail' in result