Fix gitignore pathspec checks to resolve paths against repo root

[JessicaMulein]

Summary

• Add GitRepo._resolve_path_in_repo() so relative paths are resolved under self.root, not process cwd.
• Use it in _is_gitignored_by_pathspec() when walking .gitignore rules.

Problem

_is_gitignored_by_pathspec called Path(path).resolve() on relative paths. That resolves against the caller’s working directory, not the git repo root. In multi-repo / superproject setups (session cwd at a parent repo, GitRepo rooted at a submodule), tracked files could be falsely reported as gitignored and /add would reject them.

Example: cecli submodule with whitelist .gitignore (* + !/cecli/**), cwd at superproject root, path cecli/helpers/responses.py mis-resolved and matched *.

Test plan

• Unit: relative path under repo root resolves correctly regardless of process cwd
• Manual: superproject dogfood — /add cecli/cecli/helpers/responses.py succeeds when cwd is parent repo root