[Story] Settings UI and documentation for dynamic tool loading

[edelauna]

Context

Dynamic loading introduces four new settings (`threshold`, `initialK`, `searchK`, `searchTurns`) that need a discoverable home in the MCP settings panel and clear documentation. In particular, `searchTurns: 0` is the kill-switch to disable dynamic loading entirely and should be prominently documented.

Depends on #578, #579, #580.

Developer Notes

• Add a "Dynamic tool loading" subsection to the MCP settings panel (`webview-ui/src/components/mcp/McpView.tsx`) with inputs for the four settings and inline tooltips explaining:
  • The rolling window and how `searchTurns` controls it
  • The `mcp_load` escape hatch
  • The two-tier ranker (BM25 always available; embeddings when a provider is configured)
  • `searchTurns: 0` as the kill-switch
• Update `README.md` and any existing MCP documentation to describe the feature.

Acceptance Criteria

• All four settings visible and labeled in MCP panel
• Settings round-trip: change → save → reload → value persisted
• Kill-switch (`searchTurns: 0`) documented prominently in UI tooltip and README
• README updated with dynamic loading explanation