dsh-stash: DSH's Keychain for Linking External Resource Repositories

dsh-stash is an external resource management plugin for DSH, providing keychain and access control features. It manages resource manifests through declarative configuration (supporting HTTP, local files, etc.) and enforces strict permission constraints via the `access` field, ensuring that it can be retrieved, which key is used, and what was retrieved. Credentials are stored only as references, while their values are kept in environment variables. The plugin provides 11 tools, supports ledger logging and data migration, and aims to offer a stable external data access layer for agents.