Support multiple entry files

[sobeaa]

code-slicer should support multiple entry files in one command.

Example:

code-slicer src/main.ts src/server.ts src/config.ts

Expected behavior:

• collect dependency context from all provided entry files
• de-duplicate shared dependencies in the output
• preserve a clear output order
• fail clearly if any entry file is invalid or missing
• keep current single-entry behavior unchanged