Improve diagnostics for missing bindings and sandbox requirements

[ascorbic]

Goal

Production setup failures should produce actionable errors instead of generic runtime failures.

Scope

Improve diagnostics for missing platform bindings, unavailable sandbox runners, misconfigured storage/database bindings, and plugin sandbox requirements.

Acceptance criteria

• Missing required bindings identify the binding name and feature that needs it.
• Sandbox runner failures explain whether the issue is config, platform support, or runtime startup.
• Admin/API responses avoid misleading NOT_CONFIGURED-style errors when config is present but incomplete.
• Docs link to the relevant setup instructions.
• Tests cover representative missing-binding cases.