chore: update pinned actions to their SHA-pinned releases

[kim-em]

This PR moves the pinned leanprover/lean-action to v1.6.1 (and, where used, lean-update, mathlib-update-action and intentions to their current versions). These releases pin their own internal actions, which an organization policy requiring SHA-pinned actions checks along the whole chain.

🤖 Prepared with Claude Code

[github-actions]

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[jstoobysmith]

Looks good, approved. Will merge now.

[jstoobysmith]

Opp sorry @kim-em, I think your two recent PRs have conflicted each other here.

[jstoobysmith]

awaiting-author

[jstoobysmith]

Thanks. Merging now.