Summary

Adds Node HTTP adapter support for zstd-compressed responses while keeping zstd out of the default Accept-Encoding header unless explicitly enabled through transitional config.

Linked issue

Changes

• Add capability-checked zstd decompression in the Node HTTP adapter.
• Add transitional.advertiseZstdAcceptEncoding to opt into advertising zstd support.
• Update TypeScript declarations, validation, unit coverage, and pre-release notes for the new transitional option.
• Add OpenSpec agent workflow files and repo guidance for future spec-driven changes.

Checklist

• Tests added or updated (or N/A with reason)
• Docs / types updated if public API changed (index.d.ts and index.d.cts)
• No breaking changes (or called out explicitly above)

Summary by cubic

Adds zstd (Zstandard) decompression to the Node HTTP adapter and gates zstd advertising behind a new transitional flag. Defaults unchanged; we only include zstd in Accept-Encoding when transitional.advertiseZstdAcceptEncoding is true.

Description

• Summary of changes

  • Add capability-checked zstd decompression in the Node HTTP adapter.
  • Introduce transitional.advertiseZstdAcceptEncoding; remove the old advertiseZstd request option.
  • Validate the new flag in Axios; default to false in defaults/transitional.js.
  • Use own-property check on transitional before reading the flag for security hardening.
  • Update types (index.d.ts/index.d.cts), merge rules, and pre-release notes.

• Reasoning

  • Keep behavior opt-in and predictable; ensure secure handling of transitional flags.

• Additional context

  • Applies to axios Node adapter; browsers are unaffected.

Docs

• Update /docs/ and README.md to document transitional.advertiseZstdAcceptEncoding, Node-only support, and decompression-bomb guidance.
• Update advanced request config examples to use the new transitional flag.

Testing

• Updated unit test to enable transitional.advertiseZstdAcceptEncoding and assert zstd is included in Accept-Encoding when supported.
• Optional: add an integration test to verify end-to-end zstd decompression.

Semantic version impact

• Minor: new opt-in feature with no default behavior changes; the removed advertiseZstd option was not part of a released API.

^{Written for commit ecc59c7. Summary will update on new commits. Review in cubic}