feat: add batch audio transcription provider

[ekinnee]

Summary

• Register OmniRoute as an OpenClaw media-understanding provider for batch audio transcription.
• Send explicitly selected models through OmniRoute’s multipart /v1/audio/transcriptions endpoint using OpenClaw’s public guarded helper.
• Preserve shared base-URL precedence, resolved auth, upload metadata, bounded transcript/error validation, manifest contracts, generated dist/, and batch-versus-realtime documentation.

Related issue

Closes #14

Validation

• pnpm test (13 files, 148 tests)
• pnpm build
• git diff --check
• node .github/scripts/upstream-compatibility-probe.mjs --source-contract
• Focused multipart/auth/base-URL/error tests
• Sol-medium independent diff review: no blocking P0/P1/P2 findings

Notes for reviewers

• No credentials, private endpoints, or production-only data included
• Capability and manifest ownership changes are documented when applicable
• This is batch REST transcription only; realtime or duplex voice transcription is out of scope.