fix: replace raw echo with output helpers in git/missing-release-branches

[github-actions]

Summary

• Add die, info, and success output helpers to git/missing-release-branches
• Replace all user-facing echo calls with the appropriate helper (info for progress/status, success for completion, die for fatal errors)
• Preserve internal echo "Y" inside matchingBranch which is used as a return value via command substitution — not user-facing output
• Fix A && B || C anti-pattern flagged by shellcheck (SC2015) by restructuring to a proper if/then/else
• Drop misleading || echo "Switching to Main" fallback in favour of || true since the intent is to continue even if the switch fails

Closes #646

[dnyw4l3n13]

All CI checks passing. No review comments to address. Marking PR as ready for review.

Summary of changes in this PR:

• Added die, info, and success output helper functions to git/missing-release-branches
• Replaced user-facing echo calls with the appropriate helper (info for progress, success for completion, die for fatal errors)
• Retained internal echo "Y" in matchingBranch — it is a return value via command substitution, not user-facing output
• Fixed shellcheck SC2015 (A && B || C anti-pattern) by restructuring to a proper if/then/else
• Replaced || echo "Switching to Main" with || true to preserve the continue-on-failure intent without bare echo

[credfeto]

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[dnyw4l3n13]

All CI checks passing and no outstanding review comments. PR is ready for merge.

CI summary:

• BASH linting: pass
• build-pre-release: pass
• change-log-entry-is-in-unreleased: pass
• does-not-contain-secrets: pass
• has-no-merge-conflicts: pass
• include-changelog-entry: pass
• lint-code: pass
• valid-changelog: pass
• no-ignored-files: pass
• no-merge-commits: pass
• dependency-review: pass