Follows similarly to the bash tool (and some code to connect to an agent
was extracted from it).
There are two main parts: a new agent endpoint, and then a new MCP tool
that consumes that endpoint.
Fixes https://github.com/coder/internal/issues/907
We convert `workspacesdk.AgentConn` to an interface and generate a mock
for it. This allows writing `coderd` tests that rely on the agent's HTTP
api to not have to set up an entire tailnet networking stack.
Asher