dsh-tool-time: A Time Utility Plugin for DSH

The DSH plugin `omdsh-dev/dsh-tool-time` is a zero-dependency, pure-function time processing tool designed to address error-prone time calculations in Agent tasks, such as timezone offsets, leap years, and month-end boundaries. This plugin is compatible with the DSH 0.1.2-alpha.1 system and provides a core `time` tool with four operation types: `now`, `convert`, `add`, and `diff`. It supports strict ISO 8601 subset parsing, IANA timezone conversion, UTC calendar arithmetic, and fixed-duration difference calculation. Its design emphasizes safety and determinism: input validation poses no `eval` risk, `add` operations are based on the UTC calendar rather than local wall-clock time, and `diff` only returns fixed-duration differences without supporting natural language or calendar month/year differences. The plugin establishes clear conventions for time precision; both `iso` and `unix` formats are handled with defined precision.