docs(AGENTS.md): test.sh over Makefile, ic-cdk 0.20 guidance, Rust env vars

[marc0olo]

Summary

Updates AGENTS.md with lessons learned during the Rust migration wave (PRs #1393–#1421).

Changes

test.sh replaces Makefile

• Rename the ## Makefile section to ## test.sh with a bash script template
• #!/usr/bin/env bash + set -e; plain $var syntax; no $$/\\/@ overhead
• Add delta-based balance check pattern (before/after) for idempotency across re-runs
• Update CI workflow template and README structure template to use bash test.sh
• Update migration checklist accordingly
• Topup: no longer a Makefile target — document icp canister top-up --amount 30t backend directly in the README; note that 30t is an example amount, not a fixed requirement

Image versions

• Pin all three images to 1.0.1 in the CI workflow template (motoko, rust, all)
• Note that icp-dev-env-rust:1.0.1 is the minimum for ic-cdk ≥ 0.19 (earlier images bundle candid-extractor 0.1.4 which fails on ic0::cost_call)
• Update the pending items section to show 1.0.1 as the current pinned version

Rust-specific additions

• Always use ic-cdk = "0.20"; ic-ledger-types 0.16 required for 0.20 compat (0.15 + 0.20 causes ic-cdk-executor links conflict)
• ic_cdk::export_candid!() is required at the end of every Rust canister lib.rs — without it, candid-extractor cannot find get_candid_pointer and the build fails
• ic_cdk::api::env_var_value("VAR") for runtime env vars — not env!() (compile-time) or std::env::var() (no OS env in WASM)

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