docs: add AGENTS.md with guidance for AI coding agents

[cijothomas]

Changes

Adds an AGENTS.md at the repo root targeted at AI coding agents (GitHub Copilot, Claude, Cursor, etc.). Human contributors continue to use CONTRIBUTING.md; this file just consolidates the conventions an agent must respect to keep CI green and reviews short.

What it covers

• Repository layout (workspace + the non-workspace opentelemetry-prometheus).
• Required pre-push checks — targeted cargo fmt --all, cargo clippy -p <crate> --all-targets --all-features -- -Dwarnings, and cargo test -p <crate> --all-features --lib. Includes the equivalent invocations for opentelemetry-prometheus.
• When to run the full scripts/precommit.sh — reserved for cross-crate changes, public API / trait / feature-flag changes, dependency bumps, build script / proto / codegen changes, or anything that could regress feature-gated builds. Small single-crate changes use the targeted commands.
• Formatting and lint rules (rustfmt enforced, clippy -Dwarnings, no unexplained #[allow(...)]).
• Commit / PR conventions (Conventional Commits for the title, ~500 LOC PR target, refactor-only PRs, CHANGELOG updates under ## vNext).
• Things not to do (don't push to upstream directly, don't bypass hooks, don't hand-edit Cargo.lock, don't add public API without updating allowed-external-types.toml, don't author standalone "summary of changes" markdown files).
• Pointers to the existing scripts/precommit.sh, scripts/lint.sh, scripts/test.sh, and design docs.

Why

AGENTS.md is a community convention that several editors and agent runtimes (Copilot, Cursor, Claude Code, etc.) auto-load. Without it, agents end up pushing PRs that fail trivial CI checks (e.g. rustfmt) because they don't know about scripts/precommit.sh and the workspace conventions. Having one short file at the root means contributors using agents get consistent behavior without rediscovering the setup each time.

This PR is documentation-only — no code or build changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.7%. Comparing base (f60125e) to head (6e2393d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3480   +/-   ##
=====================================
  Coverage   83.7%   83.7%           
=====================================
  Files        126     126           
  Lines      25386   25386           
=====================================
  Hits       21255   21255           
  Misses      4131    4131           

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