fix(bootstrap-repo): mapfile portability for bash 3.2 (macOS)

[jdfalk]

Summary

apply_branch_protection.sh used mapfile -t, a bash 4+ builtin. macOS ships bash 3.2 by default, so the bootstrap fails at the protection step:

apply_branch_protection.sh: line 27: mapfile: command not found

Replaces with a portable while IFS= read -r loop that builds the CONTEXTS array incrementally. Same behavior, no bash version dependency.

How discovered

While running bootstrap_repo.sh --flavor cli --mode adopt --name magnet-handler (the first real-world adoption). The earlier bootstrap-test-DELETE-ME dry-run had no pull_request workflows so the mapfile branch wasn't taken. magnet-handler has a ci workflow, which is what triggered the discovery code path for the first time.

magnet-handler bootstrap completed end-to-end after manually applying this fix:

• Repo settings ✓
• Sync from ghcommon ✓ (51 files, 11057 insertions)
• Labels ✓ (189/242, rate-limit on remainder)
• CLI flavor overlay ✓ (CHANGELOG.md seeded)
• Bootstrap commit pushed ✓
• Branch protection ✓ (with the fix; 1 required check ci)
• Registry updated ✓ (.github/bootstrapped-repos.json)
• verify_bootstrap.sh exit 0 ✓

Test plan

• shellcheck passes
• Direct invocation against magnet-handler succeeds with bash 3.2
• No other mapfile usages in the skill (grep clean)