abridge: add experimental cc_convert sidecar preset

[Meirtz]

Summary

Add an experimental external executable preset that wires a separately
installed cc_convert_sidecar process into abridge's generic Forward and
Sidecar primitives from #120.

This change only defines the process/environment contract:

• CC_CONVERT_LISTEN_ADDR
• CC_CONVERT_UPSTREAM_URL
• optional CC_CONVERT_UPSTREAM_API_KEY
• optional CC_CONVERT_LITELLM_COMPAT

It does not vendor, install, build, or publish the external binary.

This is intentionally not merge-ready under the gateway-adapter gate in
plugins/abridge/ROADMAP.md; the missing pinned artifact and required
compatibility coverage are tracked below.

Current validation boundary

• A required executable probe verifies process launch, auto-port assignment,
  health polling, and environment wiring.
• A real-binary compatibility test is present but optional when
  CC_CONVERT_SIDECAR_BIN or a PATH binary is available.
• The real-binary test calls the Forward handler directly; it does not yet
  prove the complete sandbox tunnel -> Socket.IO -> Proxy -> Forward -> real
  binary -> mock upstream path.
• SSE remains fully buffered compatibility, not incremental streaming.

No claim is made that the external translator is CI-verified or reproducible
yet.

Blocked before ready for review

• Obtain a public source URL plus immutable commit/tag, or a release
  artifact with an immutable checksum.
• Build or download that exact binary reproducibly in CI.
• Make the real translation compatibility test required.
• Add a full sandbox tunnel -> Socket.IO -> Proxy -> Forward -> real
  cc_convert_sidecar -> mock upstream round trip.
• Verify non-streaming Anthropic/OpenAI translation and buffered SSE
  compatibility through that full path.

This PR must remain Draft until all items above are satisfied.