core/agent+cli: wire max_context_chars and re-raise CancelledError

Propagate context budget from [agent] config; preserve cancel through
tool invoke for mid-turn interruption.

tests/test_config/test_agent_section.py

@@ -17,6 +17,7 @@ def test_agent_section_defaults(tmp_path: Path) -> None:
     assert store.agent_config.parallel_tools is True
     assert store.agent_config.confirm_destructive is True
     assert store.agent_config.path_denylist == []
+    assert store.agent_config.max_context_chars == 200_000  # noqa: PLR2004
 
 
 def test_agent_section_parse(tmp_path: Path) -> None:
@@ -29,6 +30,7 @@ max_tool_result_chars = 100
 parallel_tools = false
 confirm_destructive = false
 path_denylist = ["/secrets/", ".ssh/"]
+max_context_chars = 5000
 """,
         encoding="utf-8",
     )
@@ -38,3 +40,4 @@ path_denylist = ["/secrets/", ".ssh/"]
     assert store.agent_config.parallel_tools is False
     assert store.agent_config.confirm_destructive is False
     assert store.agent_config.path_denylist == ["/secrets/", ".ssh/"]
+    assert store.agent_config.max_context_chars == 5000  # noqa: PLR2004