chore: close remaining copilot cleanup items

[stefan-jansen]

Summary

• fix README clone directory command
• use keyword arguments for the legacy _update_time test call
• compute the insufficient-cash predicate once in execution handling

Verification

• uv run pytest tests/test_broker.py::TestBrokerBasics::test_equity_uses_current_mark_prices tests/test_config_wiring.py::TestShareType::test_exit_first_classifies_after_integer_rounding -q
• pre-commit run --files README.md src/ml4t/backtest/core/execution_engine.py tests/test_broker.py

[Copilot]

Pull request overview

This PR closes remaining cleanup items by improving documentation accuracy, making a legacy test call clearer/safer via keyword arguments, and refactoring order execution handling to avoid repeated “insufficient cash” predicate computation.

Changes:

• Fix README clone instructions to cd backtest after cloning ml4t/backtest.
• Update a legacy Broker._update_time(...) test call to use keyword arguments for cache inputs.
• Compute the "insufficient" in rejection_reason.lower() predicate once in ExecutionEngine._process_single_order and reuse it across branches.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
README.md	Corrects the post-clone directory name in the development quickstart commands.
tests/test_broker.py	Switches _update_time invocation to explicit keyword args for cache fields (timestamp, prices, opens, highs, lows, volumes, signals).
src/ml4t/backtest/core/execution_engine.py	Refactors execution flow to compute insufficient_cash once and reuse it for rejection/partial-fill branching.

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