Add codegraph_multi_search tool for searching across multiple indexed
projects in a single call. Architecture designed for upstream safety:

- MultiSearchState class encapsulates all multi-search logic in
  project-registry.ts (project discovery, caching, search execution)
- tools.ts only adds thin delegation layer (~76 lines vs 120+ inline)
- Session extracts all workspace folders for project discovery
- Server instructions updated with multi-search guidance

The project-registry module provides persistent project discovery via
filesystem scanning with disk cache (~/.codegraph/projects.json),
monorepo-aware depth handling, and atomic cache operations.

- multi_search falls back to the default project hint when no workspace
  folders are provided, and advertises itself read-only like the other tools
- mark projectPath required only where a tool actually takes it
- pare the multi_search default-surface test to match the explore-only surface
- add MULTIPROJ.md documenting project discovery, codegraph_multi_search,
  projectPath cross-project queries, and the tool-visibility opt-in

Co-Authored-By: Claude <[email protected]>