AGENTS.md: drop command lists that the repo shows

[rubas]

Before

`task check` is the gate: format check, compile, credo, Elixir tests, Rust
tests, zizmor. [...] `task --list` shows the rest.
- `lib/whisper_cpp/native.ex` holds the NIF stubs plus the
  `rustler_precompiled` targets, variants, and `nif_versions`.

Problem

• An agent sees these with task --list, the Taskfile, or a quick look at the tree.
• Every listed line is one more line to read and to keep true.

After

AGENTS.md keeps only what the repo cannot show: rules, traps, required order, and the commands those rules are about.