Add macOS first-mile bootstrap script

[codeforester]

Summary

• Add bootstrap.sh for blank macOS first-mile setup before handing off to basectl.
• Support source and Homebrew install modes, dry-run, explicit install directory, repo URL, branch, and Homebrew-install refusal.
• Document the bootstrap path in the README and include the new BATS suite in bin/base-test.
• Stabilize one existing repo dry-run test so it is not affected by a real sibling base-demo checkout.

Issue

Closes #401

Validation

• bash -n bootstrap.sh bin/base-test install.sh
• bats tests/bootstrap.bats tests/install.bats cli/bash/commands/basectl/tests/repo.bats
• ./bootstrap.sh --dry-run --source --install-dir /private/tmp/base-bootstrap-smoke
• ./bootstrap.sh --dry-run --brew
• env -u BASE_HOME HOME=/private/tmp/base-review-home BASE_TEST_PYTHON=/Users/rameshhp/.base.d/base/.venv/bin/python bin/base-test
• git diff --cached --check

Demo Impact

None.

Notes

The PR intentionally leaves unrelated local edits in CONTRIBUTING.md, docs/github-workflow.md, .github/pull_request_template.md, docs/base-demo-spec.md, and docs/product-vision.md out of scope.