core/agent+config: soft context budget in estimated tokens

Replace max_context_chars with max_context_tokens (default 200k est.
tokens via char/4); compact and /status use the token budget.

tests/test_cli/test_repl_commands.py

@@ -142,7 +142,7 @@ async def test_rounds(state: ReplState) -> None:
     assert state.agent.max_rounds == 40
 
 
-async def test_status_shows_context_chars(
+async def test_status_shows_context_tokens(
     state: ReplState, capsys: pytest.CaptureFixture[str]
 ) -> None:
     from plyngent.lmproto.openai_compatible.model import UserChatMessage
@@ -150,6 +150,7 @@ async def test_status_shows_context_chars(
     state.agent.messages = [UserChatMessage(content="hello")]
     assert await handle_slash(state, "/status") is True
     out = capsys.readouterr().out
+    assert "context_tokens~=" in out
     assert "context_chars=" in out
     assert "tool_result_max=" in out
     assert str(state.workspace) in out