fix: replace raw echo with output helpers in git/optimise-git

[dnyw4l3n13]

Summary

• Replace all user-facing raw echo calls in git/optimise-git with the standard output helpers (die, info, success)
• Add the output helper function definitions at the top of the script
• Fix shebang spacing (#! /bin/sh → #!/bin/sh) for consistency
• Preserve all existing script behaviour

Closes #648

Test plan

• Run shellcheck git/optimise-git — should pass with no warnings
• Verify the script executes correctly against a directory tree of git repos
• Confirm all pre-commit checks pass (they passed in CI during commit)

[dnyw4l3n13]

PR is ready for review.

All pre-commit checks (shellcheck, linters, changelog lint, TruffleHog) passed locally. CI checks build-pre-release and pull-request both passed. No review comments to address.

[credfeto]

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[dnyw4l3n13]

All CI checks have passed and there is no outstanding work.

CI summary:

• build-pre-release — pass
• lint-code (BASH super-linter) — pass
• change-log-entry-is-in-unreleased — pass
• valid-changelog — pass
• include-changelog-entry — pass
• does-not-contain-secrets — pass
• has-no-merge-conflicts — pass
• no-merge-commits — pass
• has-no-file-or-folder-case-sensitivity-issues — pass
• dependency-review — pass

No review comments to address. PR is ready for merge.