chore: Remove excel-cli skill and build scripts from VS Code extension

[sbroenne]

Context

After PR #433 unified the MCP Server and CLI with a shared named pipe service, we decided to keep them architecturally separate to avoid version conflicts (see #434 for analysis).

Decision: VS Code extension provides MCP Server only. Users who want CLI automation install the global tool separately.

Current State

• ✅ CLI executables not bundled (only Sbroenne.ExcelMcp.McpServer.exe in bin/)
• ❌ excel-cli skill still registered in chatSkills contribution point
• ❌ Build scripts still reference CLI (build:cli, build:all)
• ❌ vscode:prepublish tries to build CLI

Required Changes

1. Update package.json

Remove excel-cli skill from chatSkills contribution point (keep only excel-mcp).

Simplify build scripts - remove build:cli and build:all, clean script should only reference McpServer project.

2. Update extension README

Clarify that CLI requires separate install via global tool.

3. Update docs

• docs/INSTALLATION.md: Clarify VS Code extension vs global tool
• vscode-extension/DEVELOPMENT.md: Update build process (MCP Server only)

Benefits

• Clear separation: Extension = Copilot, Global tool = CLI
• No version conflict risk with shared service
• Smaller extension package
• Simpler build process
• No confusing "bundled but not usable" CLI

Related

• feat: Bundle CLI in VS Code extension for unified approach #434 - Original proposal to bundle CLI (closed as not planned)
• PR feat: Code generation implementation with MCP-Daemon unification #433 - MCP-Daemon unification that introduced shared service