feat(context): bound per-tool output before it enters agent history

Cap the size of every tool result so a single verbose command (recursive
find, noisy scanner, full page dump) can't pin the conversation near the
model's context window for the rest of a scan.

- New ContextSettings config group with env-tunable caps.
- Default the SDK shell tools' max_output_tokens so exec_command /
  write_stdin truncate head+tail instead of returning unbounded output.
- Bound Strix's own FunctionTool/CustomTool results (line + UTF-8 byte
  head+tail preview with a truncation notice) and cap error strings.

strix/config/__init__.py

@@ -17,6 +17,7 @@ from strix.config.loader import (
     persist_current,
 )
 from strix.config.settings import (
+    ContextSettings,
     DedupeSettings,
     IntegrationSettings,
     LlmSettings,
@@ -27,6 +28,7 @@ from strix.config.settings import (
 
 
 __all__ = [
+    "ContextSettings",
     "DedupeSettings",
     "IntegrationSettings",
     "LlmSettings",