test: certify the Public Alpha installation journey

## Goal

Prove that a new user can install, authorize, configure, use, update, and remove Diffler without repository-specific knowledge.

## Scope

- Add clean-install CI coverage for supported Node.js versions on macOS, Linux, and Windows.
- Install the packed or released npm artifact rather than importing the source checkout.
- Run `diffler --help`, `diffler doctor`, schema validation, context collection in a fixture repository, skill install/status/uninstall, and package update smoke tests.
- Mock keychain and Google boundaries in automated tests; never place live OAuth credentials in pull-request CI.
- Perform and document a manual production-OAuth smoke test using a public test account.
- Verify Claude Code and OpenCode discover the installed skill after restart and can complete the context, generation, context-bound validation, and Forms publication workflow.
- Document the three-command happy path, bring-your-own OAuth fallback, troubleshooting, data handling, privacy, revocation, uninstall, and test-Form cleanup.
- Confirm npm package, GitHub tag, release notes, and CLI version agree.

## Acceptance Criteria

- A clean user journey is documented as:
1. `npm install --global @jamestkelly/diffler`
2. `diffler auth login`
3. `diffler skill install `
- Installation and non-network commands pass on macOS, Linux, and Windows.
- At least one first-party OAuth end-to-end test publishes a graded Form from a fixture branch without a user-managed Cloud project.
- Claude Code and OpenCode each discover and execute the packaged skill.
- Logout, skill uninstall, npm uninstall, Google grant revocation, and generated Form cleanup are verified and documented.
- No test artifact exposes source diffs, answer keys, tokens, OAuth files, or user home-directory data.

## Dependencies

- Depends on #25 for the npm artifact.
- Depends on #26 for released versions.
- Depends on #27 for installer and diagnostics.
- Depends on #28 for first-party OAuth.