Self-hosted runner: inaccessible context-source repos are skipped instead of failing the whole clone

When a self-hosted runner (the claude runner binary used to run hosted or CI-style pools of sessions) prepares context sources for a session, it used to fail the whole clone if one repo couldn't be reached. Now, if a repo that isn't a work repo (it has no push_targets entry) can't be accessed through the git mount or proxy because of an authentication or permission problem, the runner logs a warning, records the skip as context_source_access_denied, and shows a step message telling the user to check their GitHub connection or ask an admin to check org access.