dsh-subagent-cwd: Add per-call cwd for DSH sub-agent calls

The DSH plugin dsh-subagent-cwd (MIT licensed) enhances sub-agent delegation capabilities based on dsh-subagent-tools. It introduces a core new per-call `cwd` parameter, allowing the specification of the working directory for a single `subagent` or `subagent_fork` call. It still supports per-call overrides for `model`, `provider`, `persona`, and `toolFilter`, as well as `@preset:` references and provider/model combination IDs. Implementing the per-call `cwd` feature relies on two in-process provider patches. The plugin comes with installation scripts for Windows (`install.ps1`) and POSIX (`install.sh`).