Harden GitHub reporting and Action CI

[thebiglabasky]

Summary

• consolidate and supersede Test unsupported command rejection #4 and Add GitHub Check name input #5 on top of the current main
• keep the Action limited to checkly test and checkly trigger
• add configurable GitHub Check names and forward GitHub reporting context metadata
• require Checkly CLI 8.15.0 or newer across every reporting mode
• add repository-specific agent instructions and release invariants

Test harness and CI

• retain fast public-behavior tests for validation, reporting decisions, and CLI argument construction
• add a credential-free integration harness with a local preflight server and fake npx process
• verify the actual preflight request, detached invocation, install command, working directory, Action outputs, step summary, and CLI exit-code propagation
• run syntax checks and both harnesses on Ubuntu and macOS
• run ShellCheck and digest-pinned actionlint on Linux
• invoke the composite Action itself in CI to catch action.yml wiring drift
• expose one stable CI aggregate check that can be required by branch protection

Compatibility and rollout

The Action has not been publicly released, so it uses one compatibility baseline instead of maintaining older reporting paths. Exact stable CLI pins must be 8.15.0 or newer. Dist-tags, ranges, canaries, and prereleases remain allowed.

The required CLI metadata support is in checkly/checkly-cli#1397 and is planned for 8.15.0. The backend support landed in checkly/monorepo#2945. The CLI release must be published before the Action is released.

Supersedes #4 and #5.

Verification

• bash -n scripts/*.sh
• node --check scripts/test-preflight-server.cjs
• shellcheck scripts/*.sh via koalaman/shellcheck:v0.11.0
• actionlint 1.7.7
• ./scripts/test.sh
• ./scripts/test-integration.sh
• git diff --check origin/main...HEAD