docs: plain English README and AGENTS.md

[rubas]

Before

CPU is available in every build except coreml (see below). Pick one accelerator per build; the precompiled Hex package ships CPU plus cuda / hipblas variants for Linux and Metal on Apple Silicon, selected via WHISPER_CPP_VARIANT:

Problem

• The README did not say which Linux targets have which GPU variant; only AGENTS.md did.
• AGENTS.md opened with a "Goal" that repeated the README, and its release steps packed several rules into one step.
• One pitfall described the release guard wrongly (see below).

After

• The README goes from what the library is, to install, usage, backends, development, and license. A small table shows the precompiled variants per target. task check and task test:integration replace the bare mix test lines.
• AGENTS.md holds the checks, rules, pitfalls, and release steps, in short sentences. The dispatch rules are their own step.
• Words: README 511 to 537, AGENTS.md 782 to 649.

Fact fixes

• AGENTS.md said the release job fails "when the ggml CPU flags contain native". release.yml checks that CMakeCache.txt has GGML_NATIVE:BOOL=OFF; the text now says that.
• The README testing section listed mix test commands; the repo gate is task check (format check, compile, lint, Elixir and Rust tests, zizmor), and task test:integration for real inference.
• Checked against the code: version 0.5.0, elixir: "~> 1.19", rust-version = "1.98", the @variants map, the six-entry release matrix, armv8.2-a+dotprod+fp16, the gfx1200 and gfx1201 arch list, the .hip_fatbin check, overwrite_files: false, the nif_versions sed, the weekly integration.yml, the [patch.crates-io] rev on vendor/whisper-rs-0.16.0-patched, issue Track upstream whisper-rs fixes; drop the vendor patches when released #26 (open), the Credo Specs and ModuleDoc settings, and the silero model file name.