[Feature]: Add ignore file generation support for GitHub Copilot

[flanny7]

Hello, I always appreciate rulesync.

Problem

Rulesync supports ignore file generation (.gitignore-style exclusion patterns) for 14 tools, but GitHub Copilot and Copilot CLI are not among them.

Evidence

In src/features/ignore/ignore-processor.ts, the supported targets list does not include copilot or copilotcli:

// Current supported targets (copilot is absent)
// augmentcode, claudecode, claudecode-legacy, cline, cursor,
// geminicli, goose, junie, kilo, kiro, qwencode, roo, windsurf, zed

As a result, rulesync generate --targets copilot --features ignore produces no output.

Proposed Solution

Add copilot (and optionally copilotcli) to the ignore processor's supported targets. The expected output file could be:

• Project mode: .github/.copilotignore (following GitHub Copilot's convention)
• Global mode: ~/.copilot/.copilotignore

This would allow users to centrally manage file exclusion patterns for Copilot alongside other tools.

Your consideration would be appreciated.