[ejohb]

Description

Add native optional acp.v1 WebSocket subprotocol negotiation to the ACP remote servers.

Fixes: #67

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (ruff format & check)
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings (basedpyright & ty)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (pytest)

Verification: Ruff format/check passed. All 7 tests in tests/acpremote/test_phase2.py, including the protocol-present and protocol-absent cases, passed. The broader tests/acpremote run stalled on its final Phase 3 test, so the full suite and basedpyright/ty checks remain unverified.