cage: update to 0.3.1 - #148

Checking PR #148: cage: update to 0.3.1 (Formalities Audit)

✅ Pull request originates from a dedicated feature branch (cage-0.3.1)

Commit 6f03a3f - cage: update to 0.3.1:

✅ Author and committer identities are valid
✅ Commit is not a merge commit
✅ Commit subject layout and length are valid: "cage: update to 0.3.1"
✅ Commit description lines adhere to width formatting rules
✅ Commit contains a valid 'Signed-off-by:' line matching author or committer
⚠️ Warning: No reference link (e.g., upstream release notes, changelog, or history URL) detected in description.
⚠️ Warning: Commit is unsigned or cryptographic signature verification failed (Reason: unsigned). Signing commits is a recommended best practice for verifying identity, but is not mandatory.