[Proposal] New Guide: Mechanical Guards for AI Agents

## Proposed Section: Mechanical Guards for AI Agents

A new guide covering a category of prompt engineering currently missing from the guide: **mechanical verification patterns** that compensate for an LLM's structural inability to verify its own NL output.

## The Gap

Current guides cover prompt design, reliability patterns, and adversarial robustness. But there's a distinct category not yet covered: deterministic, filesystem-level checks that **bypass** the LLM's reasoning layer entirely — because the LLM cannot reliably judge its own configuration integrity.

## Core Concept: The Prose Barrier

Any NL-based AI agent shares a structural constraint: its verification loop and generation loop use the same channel. An agent cannot independently verify its own NL output — it can only produce more NL claiming the first NL was correct. This is not a bug — it's an architectural property.

## What the Guide Would Cover

1. **The Prose Barrier** — why AI agents structurally cannot self-verify NL output
2. **Mechanical Gate Patterns** — filesystem checks (mtime, regex, exit codes, hook wiring) that verify config integrity without AI involvement
3. **Dual-Layer Architecture** — process-layer soft monitoring + output-layer hard blocking
4. **Causal Evidence** — config rules causally shape agent behavior (n=30 experiment, p=0.0092)
5. **Implementation Patterns** — SessionStart/PreToolUse/Stop hook designs for mechanical verification

## Evidence

- 50+ session production deployment
- Causal experiment: odds ratio 11.0, p=0.0092
- Independent structural convergence with Anthropic J-space (July 2026)
- ECC 226K★ repo: delivery-gate merged (#2378)

## Scope

~2000 words, fits as a new guide under `guides/prompts-mechanical-guards.md`. Complementary to existing `prompts-reliability.md` — focuses on the verification layer rather than prompt design patterns.

Happy to draft the full guide if maintainers are interested.

Start with guides/prompts-reliability.md and compare its coverage with the proposed guides/prompts-mechanical-guards.md. Draft the approximately 2,000-word guide covering the Prose Barrier, mechanical gate patterns, dual-layer architecture, causal evidence, and hook designs, with the cited evidence accurately represented.

Written by the indexing model from the issue text.