docs(sdk/js): rewrite README for 0.5.8-beta.1 API surface

reasons for rewrite:
- npm install instructions did not mention peer deps (consumers now
  need @noble/hashes explicitly, plus per-submodule peers after the
  0.5.8 optionalDependencies → peerDependencies change)
- getKey signature was stale (missing algorithm arg, path shown as
  required, key return type listed as hex string instead of Uint8Array)
- sign/verify section said "not yet released" though shipped in 0.5.7
- blockchain helpers used the deprecated toViemAccount / toKeypair
  variants instead of toViemAccountSecure / toKeypairSecure
- no coverage of attest, version, info.cloud_vendor / cloud_product,
  getTlsKey notBefore / notAfter / withAppInfo, secp256k1_prehashed,
  AppCompose type, or the verify+encrypt env-var flow
- migration content for the deprecated TappdClient surfaced ahead of
  the actual current API

structure: one section per capability area (keys, attestation, sign &
verify, diagnostics, blockchain, compose hash, encrypted env vars),
single example per method, compatibility table mapping each feature to
the minimum guest agent version, migration section moved to the bottom.

length drops from 508 to 291 lines with broader and more accurate
coverage. no AI-slop "Use Cases" / "Key Characteristics" filler.

Author: Leechael