feat: add configurable results directory

[huytdps13400]

Summary

• Add REASSURE_OUTPUT_DIR for keeping each package result files in a separate directory.
• Apply the directory consistently to CLI baseline/current files, measure default output, and compare JSON/Markdown reports.
• Preserve REASSURE_OUTPUT_FILE as the higher-priority explicit measurement path.
• Document the monorepo workflow and include a minor changeset.

Root cause

The result directory was hardcoded independently in the CLI, measure, and compare packages. Changing only the CLI path would still leave compare reading the original .reassure files, so the environment contract is resolved at all three package boundaries.

Testing

The new tests were observed failing before the implementation:

• measure expected custom-results/current.perf but received .reassure/current.perf
• compare exited because it still looked for .reassure/current.perf
• the Markdown regression check failed while output.md was still hardcoded

Validation after the change:

• yarn validate
• yarn build
• yarn changeset status

I also ran the built CLI end to end on Node 24 with a no-op test runner. Baseline followed by current measurement created baseline.perf, current.perf, output.json, and output.md only inside the configured package-specific directory.

Fixes #569.