feat: add E2E test suite with GitHub Actions CI

[Cannon07]

Summary

• Add comprehensive E2E test suite with 31 tests covering core plugin logic and backend-specific integration
• Plugin tests use plenary.nvim busted framework (standard Neovim plugin testing approach)
• Backend tests use shell scripts to test the real JSON → shell → Neovim RPC pipeline
• GitHub Actions CI runs on both Ubuntu and macOS with Neovim v0.10.4
• Fix ESM compatibility issue in opencode-plugin/nvim.ts (use top-level imports instead of runtime require("fs"))

Test structure

tests/
├── minimal_init.lua                  # Plenary + plugin init for Lua tests
├── run.sh                            # Main orchestrator
├── run_lua.sh                        # Plenary busted runner
├── helpers.sh                        # Shell helpers (backend tests only)
├── plugin/                           # Lua tests (plenary busted)
│   ├── changes_registry_spec.lua     # 6 tests
│   └── diff_lifecycle_spec.lua       # 5 tests
└── backends/                         # Shell integration tests
    ├── claude/
    │   ├── test_edit.sh              # 8 tests (Edit, Write, Bash, replace_all, sequential)
    │   ├── test_install.sh           # 4 tests (install, uninstall, idempotent, preserves existing)
    │   └── test_stale_socket.sh      # 2 tests (restart recovery, missing Neovim)
    └── opencode/
        ├── harness.ts                # TypeScript test harness for OpenCode plugin
        ├── test_edit.sh              # 4 tests (edit, write, bash rm, relative paths)
        └── test_install.sh           # 2 tests (install, uninstall)

Running locally

./tests/run.sh                      # all tests (plugin + backends)
./tests/run.sh plugin               # core plugin tests only (plenary)
./tests/run.sh backends             # all backend tests
./tests/run.sh backends/claude      # Claude Code backend only
./tests/run.sh backends/opencode    # OpenCode backend only
./tests/run.sh install              # fuzzy match backend tests

Test plan

• All 31 tests pass locally (macOS)
• Plugin tests use plenary busted (standard Neovim testing framework)
• Backend tests validate real hook payloads against Claude Code and OpenCode formats
• Test payloads verified against Claude Code hook documentation and OpenCode SDK source
• CI passes on Ubuntu
• CI passes on macOS

🤖 Generated with Claude Code