ci: ensure migration benchmark runs

[mrjf]

TL;DR

This fixes the migration benchmark workflow so the Python-vs-Go CLI benchmark actually runs in Actions, uploads benchmark evidence, and posts benchmark results back to the PR for each head commit. It also removes the top-level PR path filter so the workflow is created on every PR update; Crane migration branches then force the migration checks to run on every commit.

What changed

• Run the Python-vs-Go CLI benchmark immediately after building the Go CLI.
• Keep the lightweight Python scaling guards, but stop running the broad stale Python-only benchmark suite in the migration workflow.
• Fix the benchmark helper so read-only command benchmark working directories are created before subprocess execution.
• Add a clear faster/slower result column to the benchmark markdown summary.
• Trigger the migration workflow on every pull request update instead of relying on the workflow-level paths filter.
• Force migration parity and benchmark jobs to run for crane/* PR branches, even when a commit only touches orchestration or generated metadata.
• Post benchmark results as an idempotent PR comment keyed by the PR head SHA.

Validation

• /opt/homebrew/bin/actionlint .github/workflows/migration-ci.yml
• git diff --check HEAD~1 HEAD
• PYTHONPYCACHEPREFIX=/private/tmp/apm-pycache python3 -m py_compile scripts/ci/migration_cli_benchmark.py
• python3 scripts/ci/migration_cli_benchmark.py --python-bin /usr/bin/true --go-bin /usr/bin/true --json-out /private/tmp/migration-cli-benchmark-smoke.json --markdown-out /private/tmp/migration-cli-benchmark-smoke.md --repeats 1
• GitHub Actions run 26557634231: Detect Migration Changes, Python-vs-Go Parity Gate, and Migration Benchmarks all passed.
• PR benchmark comment posted for commit dbabdffd7801279bf4053a98026fa496f9505f9f.

How to test

1. Push any commit to a crane/* migration PR branch.
2. Confirm the Migration Parity and Benchmarks workflow is created for that commit.
3. Confirm Detect Migration Changes, Python-vs-Go Parity Gate, and Migration Benchmarks all run.
4. Confirm the PR receives a Migration Benchmark Results comment for that commit SHA.
5. Confirm the migration-benchmark-evidence artifact includes JSON and markdown output.

[github-actions]

Migration Benchmark Results

• Commit: dbabdffd7801279bf4053a98026fa496f9505f9f
• Run: https://github.com/githubnext/apm/actions/runs/26557634231

Migration CLI Benchmark

Max allowed Go/Python median ratio: 5.00

Command	Python median	Go median	Go/Python	Result	Return codes
--help	0.4696s	0.0013s	0.00x	369.60x faster	{'python': [0], 'go': [0]}
--version	0.4717s	0.0013s	0.00x	372.04x faster	{'python': [0], 'go': [0]}
compile --help	0.4815s	0.0012s	0.00x	398.52x faster	{'python': [0], 'go': [0]}
install --help	0.4818s	0.0013s	0.00x	372.27x faster	{'python': [0], 'go': [0]}
pack --help	0.4798s	0.0013s	0.00x	379.79x faster	{'python': [0], 'go': [0]}
audit --help	0.4688s	0.0013s	0.00x	367.88x faster	{'python': [0], 'go': [0]}
init --yes	0.4973s	0.0013s	0.00x	382.47x faster	{'python': [0], 'go': [0]}