Move the argument earlier.

Shift-left is not more testing at the start of a ticket. It is feedback while the change is still cheap to refuse.

Ready and done need evidence.

A definition of ready that nobody can fail is a slogan.

The behaviour, the risk, and one example of failure are named before implementation starts.

Done includes the check that would have caught this class of mistake, not a percentage on a dashboard.

A short example, given and then, beats a paragraph that every reader interprets differently.

Tools do not shift left. Placement does.

The author sees the failure before the branch is someone else's problem.

Seeded fixtures and stable clocks so a red run means the same thing tomorrow.

A place to look at the change without sharing a long-lived staging server that nobody trusts.

Do not start by buying a platform.

Pick one expensive failure
The class of bug that keeps returning. Write the check that would have seen it.
Put that check on the pull request
If it only runs nightly, it is not left. It is late, on a schedule.
Remove a duplicate journey
Every fast check you add should let you delete or shrink a slow one.
Review the remainder monthly
Shift-left that never retires old tests just makes a longer pipeline.

If the first serious signal is after merge, the strategy is still on the right.