docs: plain English README and AGENTS.md

[rubas]

Before

Filesystem-path inputs trust the local filesystem. They allow file,crypto,data: the network is blocked, but file is required. It is the protocol that opens the path itself, and it also lets a local HLS/DASH playlist read its sibling segment files. protocol_whitelist applies uniformly to every open libavformat performs, so there is no way to keep the top-level file open while forbidding the nested ones, and a crafted on-disk manifest can therefore still point FFmpeg at other local files via a file: reference.

Problem

• Long sentences and bold phrases made the README slow to read.
• The README listed the unsafe operations in ffi_helpers.rs, but missed two of them.
• AGENTS.md repeated the README on panics, the protocol whitelist, and the LGPL build.

After

• The README goes from what the library is, to install, usage, requirements, errors, untrusted input, safety, and development. Plain English, short sentences.
• AGENTS.md holds only the agent rules, the checks, and the steps to add an operation.
• Words: README 1408 to 1071, AGENTS.md 691 to 481.

Fact fixes

• The README list of unsafe operations missed codec_name and extradata in ffi_helpers.rs. The README now describes the module without a list that goes stale.
• The README said a panic returns {:error, %{type: "nif_panic", ...}}. That is the raw NIF map; the public API returns {:error, %Exmpeg.Error{reason: :nif_panic}}.
• task check also runs zizmor on the workflows; the README now says so and points to task --list instead of a copied task list.
• Checked against the code: version 0.6.0, the three precompiled targets and NIF 2.17, rust-version = "1.98", FFmpeg 9.0.1 and the LGPL configure flags in release.yml, the error reasons in lib/exmpeg/error.ex, the 100 ms cancel check, both whitelists in input.rs, and the glibc floors (x86_64 2.35, aarch64 2.38) with objdump -T on the v0.6.0 release tarballs.