Document Go CLI migration state and usage

[Copilot]

Description

Documents the merged Python-to-Go CLI migration state so maintainers can understand what is verified, what remains Python-backed, and how to try the Go CLI without reading issue/PR history.

• Migration state

  • Clarifies Python remains the reference/oracle.
  • Identifies the Go CLI under cmd/apm.
  • Links migration provenance: Crane Migration: Python to Go -- Full APM CLI Rewrite #78, [Crane: crane-migration-python-to-go-full-apm-cli-rewrite] #91, ci: ensure migration benchmark runs #93.
  • Records latest parity evidence: migration_score=1, progress=1, 706/706.

• Contributor usage

  • Adds copy-paste build/run commands:

    go build -o ./dist/apm-go ./cmd/apm
    ./dist/apm-go --help
    ./dist/apm-go init --yes

  • Adds local parity workflow with APM_PYTHON_BIN.
  • Adds manual Actions dispatch command.

• Benchmark evidence

  • Points reviewers to the Migration Benchmarks summary and migration-benchmark-evidence artifact.
  • Explains the Go/Python ratio column and current smoke benchmark range.

• Regression coverage

  • Adds README assertions to keep the migration status, usage commands, parity gate, and benchmark guidance documented.

Type of change

• Bug fix
• New feature
• Documentation
• Maintenance / refactor

Testing

• Tested locally
• All existing tests pass
• Added tests for new functionality (if applicable)