refactor(#100): remove outdated opencode-ntfy references

[athal7]

Removes all traces of the old opencode-ntfy plugin that this project evolved from.

Changes

• CONTRIBUTING.md: Full rewrite — removed opencode-ntfy name, old repo URL, install.sh setup, NTFY_TOPIC, bash test docs, old plugin architecture, manual release process
• install.sh: Deleted — 246-line old installer (LaunchAgent plist, manual file copy, NTFY_TOPIC setup). Project now installs via Homebrew/npm
• .devcontainer/devcontainer.json: Removed bats (old bash test framework no longer used)
• .github/workflows/ci.yml: Expanded JS syntax check from plugin/*.js to plugin/*.js service/*.js (was missing 10 of 11 files)

Closes #100