[Feature] Add AGENTS.md with minimal Rust checks and docs sync rules

[mbe24]

Problem

The repository lacks a dedicated AGENTS.md that defines clear execution rules for automation agents. We currently have contributor and CI guidance, but no single agent-focused policy document.

This leads to inconsistent agent behavior, especially around post-change Rust checks and documentation updates.

Design

Add an AGENTS.md with a minimal, explicit rule set.

For Rust code changes, require exactly:

1. cargo fmt
2. cargo clippy --all-targets --no-deps --features telemetry-otel -- -W clippy::pedantic

For behavior/usage changes, require documentation sync in the same PR:

1. README.md
2. docs/ content (including issue [Feature] Set up public documentation site with Read the Docs, MkDocs, and versioned docs #49 related updates)
3. template/skills/99problems/SKILL.md when usage/workflow changes

Scope

• Create AGENTS.md in ./agents
• Whitelist .agents/AGENTS.md in .gitignore, blacklist everything else in .agents/
• Define mandatory Rust post-change commands (minimal list above).
• Define docs-sync requirements for behavior changes.
• Keep wording consistent with current contributor/CI guidance.

Boundary

• No runtime code changes.
• No new CI jobs or workflow changes.
• No expansion beyond the two mandatory Rust commands in this iteration.

Acceptance Criteria

• AGENTS.md exists in .agents/
• Rust-change checklist contains only the two agreed commands.
• Docs-sync requirements are explicit (README.md, docs/, skill file when relevant).
• Guidance does not contradict existing project process docs/workflows.

Context

This is a lightweight process hardening change to make agent contributions more predictable and easier to review.