Add prompt hooks

[abhinav-oai]

Why

Prompt hooks let hook authors express review criteria as model-evaluated prompts while preserving Codex hook semantics. Codex should default these checks to the active thread model instead of a fixed hook model.

What

• Add type = "prompt" hook handler config, app-server API, generated schemas, and hook list support with optional model, timeout, and continueOnBlock.
• Wire prompt hooks into the hook engine for PreToolUse, PermissionRequest, PostToolUse, UserPromptSubmit, and Stop while skipping unsupported events with warnings.
• Add a core prompt-hook runner that calls the model with no tools, defaults to the thread model, and expects { "ok": boolean, "reason": string }.
• Map ok: false to Codex event-specific outcomes: deny pre-tool-use, block/continue post-tool-use according to continueOnBlock, block user prompt and stop hooks, and no-op permission requests.
• Add a Stop integration test in the style of Claude multi-criteria hooks, requiring the assistant to revise once before the prompt hook returns ok.