host: connected servers' instructions never reach the system prompt #76

An MCP server can send instructions (guidance for the model, which the spec says a host MAY add to the system prompt). chakra's host drops them. defaultPromptBuilder (host/skills.go) has two sections, cfg.Instructions and skillsSection, and nothing reads what a connected server says about itself. host/app.go creates a client.Client per server and never looks at its instructions.

Until now the client had nowhere to put them: mcpkit sent instructions inside serverInfo, off-spec, and its client never surfaced them (panyam/mcpkit#1560). panyam/mcpkit#1561 fixes the wire and adds Client.Instructions, filled on both the legacy and stateless wires.

Change: add a section after the base instructions that, for each connected server in config order, emits clientByID[id].Instructions under the same origin labelling withOriginHeader gives skill blocks. It should be recomputed each turn like skillsSection, so a late-connecting server's instructions land on the next turn. Empty instructions contribute nothing.

Test: a server built with server.WithInstructions("..."), connected through the host; Build() includes the text with its origin label, and a server without instructions leaves no gap.

Waiting on: mcpkit v0.8.2, the first tag carrying #1561. Bump github.com/panyam/mcpkit from v0.7.2 to that tag in the same PR. It is also a breaking release for core.ServerInfo.Instructions, which chakra does not use, so the bump should be version-only otherwise.

Relevant to #57: an agentchat session pointed at a server like declaire currently never tells the model what that server is for.