docs: plain-English README and AGENTS.md

[rubas]

Before

• Whisper timestamp tokens are absent from the tokenizer vocab. Their base ID
  is no_timestamps_id + 1, which matches faster-whisper. See

(AGENTS.md ended there, mid-sentence, since #45.)

Problem

• AGENTS.md was cut off and pointed at code that has moved; it did not link the release runbook.
• The README had a speed claim we do not measure, said the Hex package ships the NIFs, and split device selection over three places.

After

• README: what the library is, then install, models, usage, options, errors, backends and devices, and testing, in short active sentences.
• AGENTS.md: builds and tests, a link to docs/release.md, and the design decisions, with each code pointer checked.

Fact fixes

• AGENTS.md: finished the cut sentence with SpecialTokens::resolve in tokens.rs.
• AGENTS.md: the English-only prompt branch is in PromptParts::build (tokens.rs), not transcribe.rs::transcribe_many.
• AGENTS.md: noted that config/config.exs forces a source build in this repo, which is why the first compile takes 10 minutes.
• AGENTS.md: linked docs/release.md, which holds the release runbook since docs: rewrite AGENTS.md and move the release runbook to docs/ #45.
• README: the NIFs come from the GitHub release, not the Hex package.
• README: dropped "4-8x faster than openai-whisper"; faster-whisper itself claims up to 4x, and we did not measure it.
• README: listed dnnl among the source-build features; merged the three device-selection passages into one section.