Extract shared artifact-removal helpers to reduce duplication in Configurator

[bguidolim]

Context

PR #234 added removeNewlyExcludedComponentArtifacts() and reconcileStaleArtifacts() to handle convergence gaps. Both methods duplicate the same per-artifact-type removal patterns (MCP servers, files, brew packages, plugins, gitignore entries) that already exist in unconfigurePack().

Problem

Three methods in Configurator.swift perform artifact removal with near-identical patterns:

• unconfigurePack() — removes all artifacts for a whole pack
• removeNewlyExcludedComponentArtifacts() — removes artifacts for newly-excluded components
• reconcileStaleArtifacts() — removes stale artifacts from previous sync

Each reimplements: ResourceRefCounter checks for brew/plugins, GitignoreManager removal loops, exec.removeMCPServer() calls, strategy.removeFileArtifact() calls. ~100-120 lines of duplicated removal logic.

Proposed Solution

Extract per-artifact-type removal helpers:

• removeMCPArtifact(name:scope:executor:) -> Bool
• removeFileArtifact(relativePath:) -> Bool
• removeBrewArtifact(package:packID:refCounter:executor:) -> Bool
• removePluginArtifact(name:packID:refCounter:executor:) -> Bool
• removeGitignoreArtifact(entry:) -> Bool

All three callsites can then use these helpers, reducing duplication and ensuring removal logic stays consistent.

Notes

The three methods operate at different abstraction levels (component definitions vs artifact records), so a full unification isn't practical. Per-type helpers are the right granularity.

Discovered during /simplify code review of PR #234.