Add GitHub Copilot IDE support

Problem Statement

Users report poor experiences when using swamp in VS Code with GitHub Copilot. The root cause is that swamp has no Copilot support at all — there is no swamp repo init --tool copilot option and no .github/copilot-instructions.md is generated.

Currently supported IDEs: Claude Code, Cursor, Kiro/Windsurf, OpenCode, Codex. GitHub Copilot is missing entirely.

Proposed Solution

Add a copilot tool option to swamp repo init and swamp repo upgrade that generates:

• .github/copilot-instructions.md — Copilot's standard instructions file, containing the same core swamp rules and skill descriptions generated for other IDEs
• Skills directory mapping for Copilot (likely .github/skills/ or reusing an existing convention)
• Any Copilot-specific settings or hook equivalents if applicable

The implementation should follow the existing pattern in repo_service.ts where a shared instruction body is wrapped with tool-specific frontmatter/formatting.

Alternatives Considered

• Asking users to manually create .github/copilot-instructions.md — poor UX and error-prone
• Only supporting Claude Code — limits swamp's reach to users who prefer other IDEs

Context

Two separate user reports of poor experience: one in Cursor (which has support but may need investigation) and one in VS Code + Copilot (which has zero support). The Copilot gap is the more critical issue since there is no workaround.