[codex] split command adapters and enforce public contract checks

[6uclz1]

What changed

This PR refactors the CLI command layer into smaller, focused modules and adds explicit public-contract guardrails so internal refactors can proceed safely without silently changing user-facing behavior.

Issue and user impact

The command implementation had grown into large, multi-responsibility modules (notably arrangement.py and track.py) with transport, parsing, and command adaptation concerns mixed together. That made routine changes harder to reason about and increased the chance of accidental regressions in CLI behavior, JSON output shape, protocol handling, and exit-code semantics that users depend on in scripts.

Root cause

Public/stable boundaries were documented only implicitly, and there was no automated snapshot gate to detect contract drift. At the same time, command adapters accumulated too many responsibilities in single files, which increased coupling and made intent-preserving changes expensive and error-prone.

Fix

The refactor splits command responsibilities into dedicated internal modules (for arrangement/track subdomains and shared command-runner utilities), while keeping the public command surface unchanged. It also introduces explicit contract enforcement by adding a generated public snapshot (tests/snapshots/public_contract_snapshot.json), a contract check module, and a regeneration tool (tools/update_public_contract_snapshot.py).

In addition, client/backend responsibilities are separated (src/ableton_cli/client/backends.py), remote/contract paths and error mapping were tightened, and contributor guidance now documents the public contract boundary and snapshot update workflow.

Validation

The following checks were run on this branch:

• uv run python -m ableton_cli.dev_checks
• uv run ruff check .
• uv run ruff format --check .
• uv run pytest

All checks passed (including 515 passed in pytest).