feat(coding-agent): wire spawn cwd, isolated workspaces and push authorization (R4, R12, R6) master
- `rlm.spawn(..., cwd=...)` and the runtime-created isolated checkout now reach the child through `CreateRlmSubagentRuntimeOptions.cwd`, so an isolated child really starts in `<repo_root>/.worktrees/<name>` instead of the parent's cwd (the R4 default still applies when no cwd is given) - `rlm.spawn(..., allow_push=True)` reaches the child as `RLM_ALLOW_PUSH=1`, which is the only thing that lifts the git/jj publish guard for a child; the kwarg and the workspace value are validated before any allocation - the prompt now states the two corrections a model needs when a guard fires: `line_hint=` for a non-unique `old_str`, and `RLM_ALLOW_PUBLIC_LISTEN=1` when a public bind really is wanted