Add state-machine modelling guidance

[leynos]

Summary

• add concise Rust guidance for recognizing correlated state fields that should
  become an ADT
• distinguish caller-driven typestate from runtime input- and event-driven ADTs
• require transition owners to encapsulate mandatory finalization and cleanup
• retain booleans for genuinely independent observations
• add rendered-template assertions for the new policy wording

Scope

This changes only the templated AGENTS.md and its parent-template regression
test. It does not prescribe typestate universally or add a state-machine
framework.

Validation

• reviewed the two-file branch diff through the GitHub connector
• added assertions that render AGENTS.md and check each stable policy phrase
• no local repository checkout was available in this session; draft PR CI should
  run the template, Markdown, and spelling checks

Summary by Sourcery

Add state-machine modelling guidance and regression coverage to the templated project instructions.

New Features:

• Add Rust guidance for modelling correlated or mutually exclusive state with appropriate algebraic data types and typestate patterns.

Enhancements:

• Clarify when to use caller-driven typestate versus runtime state machines and retain booleans for independent observations.
• Require state owners to encapsulate transitions, mandatory finalization, and cleanup.

Documentation:

• Update the templated AGENTS.md guidance with state-machine modelling policy.

Tests:

• Add rendered-template regression assertions covering the new state-machine guidance.

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[sourcery-ai]

Reviewer's Guide

The PR adds concise state-machine modelling guidance to the templated AGENTS.md, covering ADT selection, typestate boundaries, transition ownership, cleanup, and appropriate boolean use, with regression assertions verifying the rendered policy text.

File-Level Changes

Change	Details	Files
Add Rust guidance for identifying and designing state-machine representations.	Recommend ADTs for mutually exclusive or correlated state data, including state-specific payloads. Distinguish typestate for caller-selected finite workflows from runtime ADTs and stacks for input- or event-driven and unbounded state. Require state owners to encapsulate transitions, semantic results, mandatory finalization, and cleanup. Preserve booleans when observations are genuinely independent.	template/AGENTS.md.jinja
Extend rendered-template regression coverage for the new state-machine policy.	Assert that rendered AGENTS.md contains each stable state-modelling and cleanup phrase.	tests/test_template/test_policy_guidance.py

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