Action hooks for runtime environment setup / teardown #1269

Problem

The isolated DB / object-cache boundary lives in Runtime_Environment_Setup (set_up() / clean_up()), used from admin AJAX and CLI_Runner. There is no supported hook at that boundary for integrations that need to observe or react to setup/teardown itself.

Proposed solution

Fire do_action() from includes/Checker/Runtime_Environment_Setup.php (names aligned with existing wp_plugin_check_*), at these four moments:

1. Before set_up() mutates state
2. After set_up() completes for this request
3. Before clean_up() runs
4. After clean_up() completes

Optionally pass one small, documented argument to each hook (e.g. an array of stable flags such as early-exit) so listeners get context without a wide, fragile public surface.

Benefits

• One extension surface for admin and CLI.
• Observability and automation (metrics, assertions) without parsing AJAX.
• Matches WordPress patterns; low risk because logic stays centralized.

Implementation notes

Hooks run while DB and drop-in state change—easy to break things if misused. Document which hooks run when set_up() / clean_up() exit early; pass few stable args; add @since on new hooks.