perf: use shallow copy for args/test_cfg in worktree mode

[KRRT7]

Summary

Replace copy.deepcopy() with copy.copy() in mirror_paths_for_worktree_mode().

Part of #2132 — targeted E2E pipeline performance improvements.

Problem

mirror_paths_for_worktree_mode() deep-copies args (argparse.Namespace) and test_cfg (TestConfig). Deep copy recursively traverses every attribute — expensive for objects with many fields. But both objects contain only immutable types (Path, str, bool) and all fields are reassigned as new objects after copying.

Solution

copy.copy() — creates a new object with shared references to the same immutable values. Safe because every field is subsequently overwritten with a new Path/str, so the original is never mutated through the copy.

Changes

• codeflash/optimization/optimizer.py: copy.deepcopy → copy.copy (2 lines)
• tests/test_worktree.py: 3 new tests verifying copy independence

Test plan

• uv run pytest tests/test_worktree.py -v — all 4 tests pass
• Tests verify originals are preserved after mirroring and mutation