[Story] mcp_load native tool (dynamic tool attachment escape hatch) #579

Context

When dynamic loading is active (#578), the model may need a tool that wasn't in the initial top-K. `mcp_load` is the escape hatch: the model calls it with a natural-language query, and Zoo Code attaches the matching tools to the next API turn.

Depends on #578. Part 2 of 2 — see also #578.

Developer Notes

• New native tool definition `src/core/prompts/tools/native-tools/mcp_load.ts`: schema `{ query: string }`.
• New tool handler `src/core/tools/mcpLoadTool.ts`: calls `ToolRouter.search(query, scopedTools, searchK)`, adds results to `Task.attachedMcpToolNames`, returns the newly attached tool names + one-line descriptions. Idempotent: re-attaching an already-attached tool returns the current list without churn.
• Register `mcp_load` in the native tool list; include it in the API tools array whenever dynamic loading is active (i.e. threshold was exceeded).
• Add setting `mcp.dynamicLoad.searchK` (default 10).

Acceptance Criteria

• `mcp_load` appears in the API tools array when and only when dynamic loading is active
• Calling `mcp_load("specific query")` adds matching tools to `Task.attachedMcpToolNames`
• Subsequent API call includes newly attached tools
• Re-calling with the same query is idempotent (no duplicate entries)
• Setting `mcp.dynamicLoad.searchK` registered in schema
• Integration test: model calls `mcp_load` → correct tools attached on next turn