fix: normalize trailing slashes in excluded directories

[00200200]

excluded-directories entries such as vendor/ and ./vendor/ never match because the matcher appends another slash. Normalize trailing separators before matching while retaining path-component boundaries. Tests exercise root and nested directories, similarly prefixed names, and empty patterns.

Validation: python -m pytest claudecode/test_github_client.py -q (============================== 29 passed in 0.55s ==============================). The regression tests fail on the unchanged main branch. Tests use local fixtures; no paid API calls.

Prepared with Codex assistance; the final diff and regression behavior were checked locally.