AGENTS.md: drop command lists that the repo shows

[rubas]

Before: AGENTS.md lists commands and files that the repo already shows, for example:

task check is the default gate: format check, compile with --warnings-as-errors, credo strict, ...
native/exmpeg_native/src/ holds one file per operation plus shared modules ...: input.rs ..., atomic_output.rs ...

Problem

• An agent finds these with task --list, the Taskfile, or a quick look at the tree.
• Every such line is upkeep, and it goes stale when the Taskfile changes.

After: AGENTS.md keeps only what the repo cannot show: rules, required order, and traps. A line that holds a rule keeps the rule and the command it is about.