feat: upstream unreachableTactic linter

[wkrozowski]

Read this section before submitting

• Ensure your PR follows the External Contribution Guidelines.
• Please make sure the PR has excellent documentation and tests. If we label it missing documentation or missing tests then it needs fixing!
• Include the link to your RFC or bug issue in the description.
• If the issue does not already have approval from a developer, submit the PR as draft.
• The PR title/description will become the commit message. Keep it up-to-date as the PR evolves.
• For feat/fix PRs, the first paragraph starting with "This PR" must be present and will become a
  changelog entry unless the PR is labeled with no-changelog. If the PR does not have this label,
  it must instead be categorized with one of the changelog-* labels (which will be done by a
  reviewer for external PRs).
• A toolchain of the form leanprover/lean4-pr-releases:pr-release-NNNN for Linux and M-series Macs will be generated upon build. To generate binaries for Windows and Intel-based Macs as well, write a comment containing release-ci on its own line.
• If you rebase your PR onto nightly-with-mathlib then CI will test Mathlib against your PR.
• You can manage the awaiting-review, awaiting-author, and WIP labels yourself, by writing a comment containing one of these labels on its own line.
• Remove this section, up to and including the --- before submitting.

---

This PR <short changelog summary for feat/fix, see above>.

Closes <RFC or bug issue number fixed by this PR, if any>

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 2b5d154a4cfbd4ab1a01a2c74a95172a2c1969a4 --onto 3c6317b6d77a565b4217532d1190ac6955dba842. You can force Mathlib CI using the force-mathlib-ci label. (2026-04-29 17:45:12)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 2b5d154a4cfbd4ab1a01a2c74a95172a2c1969a4 --onto 3fc99eef102549c743c6c63547983319bfae6f01. You can force reference manual CI using the force-manual-ci label. (2026-04-29 17:45:14)