fix: correct OpenCode and Copilot MCP configs

[abdul3asith]

Summary

Addresses part of Issue #29.

This PR corrects MCP installation for GitHub Copilot CLI and OpenCode.

• Writes Copilot CLI configuration to
  ~/.copilot/mcp-config.json
• Adds Copilot's expected local MCP fields
• Writes OpenCode configuration to
  ~/.config/opencode/opencode.json
• Uses OpenCode's expected mcp schema and command array
• Preserves unrelated settings and MCP servers
• Supports idempotent installation and targeted uninstallation
• Adds regression tests for both integrations

The remaining integrations mentioned in #29 are intentionally left for
follow-up work after manual verification.

Test plan

• cargo test --all passes
• cargo clippy --all-targets passes
• Tested manually (described below)

Additional automated verification:

• cargo test -p infigraph-cli passes
• cargo fmt --all --check passes
• All 12 config_targets tests pass

Manual verification:

• Ran infigraph install using an isolated home directory.
• Confirmed GitHub Copilot CLI discovered Infigraph through
  ~/.copilot/mcp-config.json.
• Launched Copilot CLI from a TypeScript repository and successfully
  queried the repository using Infigraph MCP tools.
• Confirmed OpenCode discovered Infigraph through
  ~/.config/opencode/opencode.json.
• Launched OpenCode from a TypeScript repository and successfully called
  the Infigraph list_projects, index_project, and search tools.
• Confirmed both clients returned TypeScript function results from the
  indexed repository.