[Story] Threshold gate and Task.attachedMcpToolNames #578

Context

This wires `ToolRouter` (#577) into the task build path: when the scoped MCP tool count exceeds a threshold, only the top-K tools ranked by the initial task text are injected into the API call. The `mcp_load` escape hatch (#579) is added in the next story; this story establishes the data structure and the gate.

Depends on #577. Part 1 of 2 — see also #579.

Partially closes: RooCodeInc/Roo-Code#11990.

Developer Notes

• Add `attachedMcpToolNames: Set` to `Task` state (persisted with task history so the set survives re-attach).
• In `src/core/task/build-tools.ts`: after `getMcpServerTools()` produces `mcpTools`, check `mcpTools.length` against `mcp.dynamicLoad.threshold` (new setting, default 50; replaces the existing `MAX_MCP_TOOLS_THRESHOLD = 60` in `packages/types/src/mcp.ts`).
  • Under threshold: existing path unchanged.
  • Over threshold: call `ToolRouter.attachInitial(taskText, scopedTools, initialK)`, initialise `Task.attachedMcpToolNames`, filter the API `tools` array to only attached names.
• Add setting `mcp.dynamicLoad.initialK` (default 30).
• Remove the `too_many_tools_warning` block from `src/core/task/Task.ts:1811` — the new flow supersedes it (the UI replacement comes in [Story] Replace TooManyToolsWarning with DynamicToolsStatus indicator #581).

Acceptance Criteria

• `Task.attachedMcpToolNames` persisted and restored on re-attach
• Under threshold: API tools array unchanged
• Over threshold: API tools array contains only attached tools + native tools (no `mcp_load` yet — that's [Story] mcp_load native tool (dynamic tool attachment escape hatch) #579)
• `too_many_tools_warning` message removed from Task
• New settings `mcp.dynamicLoad.threshold` and `mcp.dynamicLoad.initialK` registered in schema
• Integration test: 100 mock MCP tools → initial API call has ≤ initialK MCP tools + native tools