ci: add daily master tags

[kim-em]

This PR adds a GitHub Actions workflow that creates a master-YYYY-MM-DD tag from master each day at 10:00 UTC.

Downstream repos that depend on mathlib can pin to these tags instead of waiting for the monthly toolchain release tags.

The workflow is idempotent (skips if the tag already exists) and can also be triggered manually via workflow_dispatch.

🤖 Prepared with Claude Code

[github-actions]

PR summary 32fe3f9354

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

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

You can run this locally as follows

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

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for scripts/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/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).

[bryangingechen]

Suggested change

	- uses: actions/checkout@v4
	- uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2

Thanks!
bors d+

[mathlib-bors]

✌️ kim-em can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.