v0.6.0 — OpenAPI 3.1 + KHDP Open API for AI Agents

Changed

• API spec is now OpenAPI 3.1, bundled into the Worker at
  openapi/v1.json and served at https://khdp.ai/openapi.json
  with a Redoc-rendered companion at https://khdp.ai/docs. The
  prose docs/REST_API.md is removed.
• SDK internal paths switched to short canonical —
  cli_datasets.py, cli_submissions.py, and tests now call
  /datasets/* and /submissions/* instead of the legacy
  /open/datasets/* and /open/dataset-submissions/*. Consequence:
  SDK 0.6.0 only works against https://khdp.ai/v1; users who pin
  KHDP_API_BASE to the legacy https://khdp.net/_api should stay
  on SDK 0.5.x until that base is deprecated.
• AGENTS.md now tells agents to fetch the OpenAPI spec from
  https://khdp.ai/openapi.json at session start instead of reading
  the deleted REST_API.md.
• 5 language READMEs, 5 quickstarts, examples (Python scripts +
  Colab notebook), and the examples/README.md all point at
  https://khdp.ai/docs and https://khdp.ai/openapi.json.
• User-Agent bumped to khdp/0.6.0.

Added

• worker/src/index.ts imports openapi/v1.json and serves it at
  /openapi.json + a small Redoc HTML at /docs and /docs/.
  Vitest covers both routes.

Removed

• docs/REST_API.md (replaced by OpenAPI 3.1 spec).
• GITHUB_REST_API_RAW env var from worker/wrangler.toml.
• /REST_API.md route from the Worker.