fix: keep run and test commands in the requested cwd

[rodboev]

/run and /test already pass aider's selected cwd into run_cmd(), but the pexpect path starts the shell with -i -c. Interactive startup files run first, so a .bashrc with cd ... can move the command before it executes.

This keeps the existing pexpect streaming path, but runs shell commands with -c instead of -i -c so the requested cwd remains authoritative.

Closes #4270

Issue #4270's .bashrc cd reproduction shaped the regression test for this fix.

Tests:

• rtk python -m pytest tests/basic/test_run_cmd.py -v
• rtk pre-commit run --all-files

Latest run:

• rtk python -m pytest tests/basic/test_run_cmd.py -v: passed
• rtk pre-commit run --all-files: passed