Add multi-project integration test for shared global packs

[bguidolim]

Problem

Issue #121 added end-to-end lifecycle integration tests covering 9 scenarios, but no test exercises two projects sharing global packs with different per-project configurations. This is the last untested cross-component interaction path identified in #121.

What's missing

When two projects share global packs (brew packages, plugins, MCP servers with user scope), the reference counting system (ResourceRefCounter + ProjectIndex) must ensure:

• Global resources are not removed when one project deselects a pack but another still uses it
• mcs pack remove correctly discovers all affected scopes and unconfigures each
• ProjectIndex accurately tracks which projects use which packs

No existing test exercises this multi-scope interaction end-to-end.

Proposed Solution

Add a multi-project integration test scenario:

Scenario: Two projects sharing a global pack

1. Create two temp projects (A and B) under the same sandbox home
2. Create a pack with a brew package, MCP server, and template
3. Configure project A with the pack → verify artifacts installed, project index updated
4. Configure project B with the same pack → verify project index has both entries
5. Remove the pack from project A → verify:
   • Global resources (brew, plugins) are NOT removed (project B still uses them)
   • Project A's local artifacts (template sections, settings, hooks) ARE removed
   • Project index still has project B's entry
6. Remove the pack from project B → verify:
   • Global resources are now removed (ref count = 0)
   • Project index has no entries for this pack

Implementation notes

• Builds on the LifecycleTestBed infrastructure from Add end-to-end integration tests for full pack lifecycle #121
• Needs two Configurator instances (one per project) sharing the same Environment(home:) and MockClaudeCLI
• ResourceRefCounter reads ProjectIndex to decide whether to remove shared resources — this is the critical cross-project interaction to verify
• Keep tests hermetic: sandbox home, no real ~/ paths

References

• PR [#121] Make DoctorRunner and checks accept injectable Environment #256, [#121] Add sandbox tests for core doctor checks #257, [#121] Add DoctorRunner integration test harness #258, [#121] Add end-to-end lifecycle integration tests #259 — Add end-to-end integration tests for full pack lifecycle #121 implementation (stacked PRs)
• Sources/mcs/Install/ResourceRefCounter.swift — two-tier reference counting
• Sources/mcs/Core/ProjectIndex.swift — cross-project pack tracking
• Tests/MCSTests/LifecycleIntegrationTests.swift — existing lifecycle test infrastructure