fix: make ci-readiness-check.sh compatible with bash 3.2

[tylerbutler]

Description

The ci-readiness-check.sh script failed to run on macOS because it used bash 4+ features that aren't available in the bash 3.2 that ships as the default /bin/bash on macOS.

Changes

• Replaced the declare -A associative array (used to dedupe changed packages) with a newline-delimited string deduplicated via sort -u.
• Replaced mapfile -t with a while read loop to populate the PACKAGES array.

Validation

Verified the script passes a syntax check (bash -n) under the stock macOS bash:

GNU bash, version 3.2.57(1)-release (arm64-apple-darwin25)
syntax OK

[github-actions]

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• Correctness — logic errors, race conditions, lifecycle issues
• Security — vulnerabilities, secret exposure, injection
• API Compatibility — breaking changes, release tags, type design
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• Testing — coverage gaps, hollow tests

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[github-actions]

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