ci: self-heal nightly-testing-green after a history rewrite

[kim-em]

This PR makes the nightly-testing-green update in nightly_detect_failure.yml recover from a history rewrite instead of freezing. The branch is advanced with a plain fast-forward push, which is rejected once nightly-testing history diverges from its tip (for example a toolchain reset to an rc). #38847 dropped the original git push --force and deferred recovery to a daily force-push job that does not exist, so the branch stuck on v4.31.0-rc1 for weeks until it was force-pushed by hand.

Fast-forward when possible, and otherwise force-push to recover, but only when the tested commit is strictly newer than the current tip by committer date, so an out-of-order CI completion can never roll the branch backwards. nightly-testing-daily keeps its fast-forward-or-skip behavior, and the stale comment referencing the nonexistent force-push job is corrected.

🤖 Prepared with Claude Code

[github-actions]

PR summary 9b1c62f014

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit 9b1c62f).

• +0 new declarations
• −0 removed declarations

No declaration differences.

---

No changes to strong technical debt.

No changes to weak technical debt.

Current commit 9b1c62f014
Reference commit 9e7b1c1166

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

---

⚠️ Workflow documentation reminder

This PR modifies files under .github/workflows/.
Please update docs/workflows.md if the workflow inventory, triggers, or behavior changed.

Modified workflow files:

• .github/workflows/nightly_detect_failure.yml