fix: initialize srcs path before get_codes in incremental mode

[octo-patch]

Fixes #2007

Problem

In incremental mode (config.inc = True), WriteCode.run() calls get_codes() which internally accesses project_repo.srcs. The srcs property raises ValueError: Call with_srcs first. when _srcs_path has not been initialized on the ProjectRepo object.

This occurs when the project's source directory does not yet exist at the start of incremental development — ProjectRepo.__init__ calls code_files_exists(), which returns early without calling with_src_path() if the source directory is absent. The Engineer._think() fallback that calls with_src_path() covers most cases, but there are scenarios (e.g., fresh incremental projects or WriteCode used outside the Engineer role) where _srcs_path can remain None when WriteCode.run() is called.

Solution

Add a defensive guard in WriteCode.run() that initializes _srcs_path via get_project_srcs_path() when src_relative_path is None, before calling get_codes() in the incremental branch. This mirrors the existing initialization in Engineer._think() and QaEngineer._think().

Testing

The fix is a targeted guard that only fires when _srcs_path is not already set, so it does not affect the normal code path. The existing test suite (test_write_refined_code, test_get_codes) continues to cover incremental mode behavior.