Support MCP Tasks

**What problem would this solve?**
* goose doesn't yet support the new Tasks primitive in MCP https://modelcontextprotocol.io/extensions/tasks/overview
* They are included in the 2026-07-28 spec as an official extension (like Apps), and seem on a direction to go into the core in a future version of MCP. We should support them.

**What would a good outcome look like?**
* Tasks and their full lifecycle are supported from an MCP client perspective in goose
* Long-running operations can be modeled by this

**Possible approaches**
* Use the existing rmcp support for them in the 3.x copy of the library and add the goose-side handling and light UI for interacting with tasks

**Additional context**
N/A

- [x] I have verified this does not duplicate an existing feature request

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).

The issue mentions using existing rmcp support in the 3.x library copy. Start by examining the MCP tasks extension spec and locating the current MCP client implementation in the goose codebase. Look for how other MCP primitives (like Apps) are handled to understand the pattern. The goal is to add task lifecycle support and a light UI for interaction.

Written by the indexing model from the issue text.