v2 refactor: @devrev/airsync-sdk (rename, state/adapter split, emit-from-return) - #208

Description

This PR is the v2 rebuild of the SDK, delivered as three single-purpose commits
(refactor / test / docs) for commit-by-commit review. Breaking changes are intended.

• Rename & branding — @devrev/ts-adaas → @devrev/airsync-sdk (2.x);
  AirdropEvent/AirdropMessage → AirSyncEvent/AirSyncMessage (hard rename).
• Adapter & state split — WorkerAdapter → ExtractionAdapter + LoadingAdapter;
  State → ExtractionState + LoadingState; on-disk state is now a
  { connectorState, sdkState } envelope with a v1→v2 read-time migration.
• emit-from-return — emit() is now SDK-internal; task/onTimeout return a
  TaskResult (success / progress / delay / error) and the SDK emits once.
• Cleanup — removed the deprecated event-type translation layer, dead
  migrateProcessedAttachments/createAdapterState shims, and src/deprecated/**.
• Migration guide & skill — added MIGRATION.md (complete v1→v2 spec, shipped in the
  npm tarball) and the migrate-v2 skill that executes it.

Connected Issues

• https://app.devrev.ai/devrev/works/ISS-306033
• https://app.devrev.ai/devrev/works/ISS-312163
• https://app.devrev.ai/devrev/works/ISS-317429
• https://app.devrev.ai/devrev/works/ISS-317430
• https://app.devrev.ai/devrev/works/ISS-330637

Checklist

• Tests added/updated and ran with npm run test OR no tests needed.
• Ran backwards compatibility tests with npm run test:backwards-compatibility.
• Code formatted and checked with npm run lint.
• Tested airdrop-template linked to this PR.
• Documentation updated and provided a link to PR / new docs OR no docs needed.