feat(installer): GitHub Copilot targets — VS Code, Copilot CLI, JetBrains

Adds three new installer targets so `codegraph install` can wire the
MCP server into GitHub Copilot surfaces:

- copilot-vscode: .vscode/mcp.json (local) or the VS Code User-dir
  mcp.json (global), JSONC-surgical edits, `--path` pinned via
  ${workspaceFolder} for global installs
- copilot-cli: ~/.copilot/mcp-config.json
- copilot-jetbrains: github-copilot config dir (XDG / %LOCALAPPDATA%)

Detection, install, uninstall, and --print-config are covered for all
three in installer-targets.test.ts, including platform-specific path
resolution.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,9 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### New Features
+
+- GitHub Copilot is now a supported agent: `codegraph install` can configure Copilot Chat in VS Code (`copilot-vscode`), the GitHub Copilot CLI (`copilot-cli`), and the Copilot plugin in JetBrains IDEs (`copilot-jetbrains`). Installed Copilot surfaces are auto-detected like every other agent, existing MCP server entries in their config files are preserved, and `codegraph uninstall` reverses the setup cleanly. Restart VS Code or your JetBrains IDE after installing so Copilot picks up the server.
 
 ## [1.5.0] - 2026-07-21