planner_agent includes test files in affected_files causing coder to patch wrong file

[VinaySampath14]

When planner_agent identifies affected files, it sometimes includes test files
(e.g. tests/test_utils.py) in the affected_files list. The coder_agent then
attempts to patch the test file instead of the source file, producing an
incorrect fix.

Expected: affected_files should only contain source files, not test files
Actual: test files included, coder patches wrong file

File: src/agents/planner_agent.py