feat: add GitHub Copilot CLI installer target

[RubenGomGar]

feat: add GitHub Copilot CLI installer target

Description

Adds GitHub Copilot CLI as the fifth installer target alongside Claude Code, Cursor, Codex CLI, and opencode.

Changes

• New file: src/installer/targets/copilot.ts - CopilotTarget implementation
• Updated: TargetId type to include 'copilot'
• Updated: ALL_TARGETS registry to include copilotTarget
• Tests: 10 new Copilot-specific edge case tests + full coverage in parameterized contract tests
• Docs: Updated README badges and supported agents list
• Changelog: Documented new feature in [Unreleased] section

Implementation Details

The target follows the same architectural pattern as existing targets:

• Global-only support (like Codex) - no project-local config concept
• MCP server config at ~/.copilot/mcp-config.json
• Agent instructions at ~/.copilot/AGENTS.md
• Copilot-specific fields: "type": "local" and "tools": ["*"]"
• Sibling preservation: Existing MCP servers are preserved during install/uninstall
• Idempotent operations: Re-running install reports "unchanged" when already configured

Testing

• ✅ All new Copilot tests pass (10/10)
• ✅ All parameterized contract tests pass for Copilot
• ✅ Existing test suite: 622/631 passing (9 pre-existing failures unrelated to this PR)
• ✅ Manual verification completed:
  • Fresh install → files created correctly
  • Idempotency → re-run reports "unchanged"
  • Sibling preservation → aspire MCP server preserved
  • Uninstall → codegraph removed, siblings preserved
  • Print config → valid JSON output

Installation Example

# Global install
npx @colbymchenry/codegraph install --target=copilot --location=global

# Verify
gh copilot mcp list
# Output: codegraph (local)

[blueheron786]

Hoping to seeing this merged!