fix(ci): crates.io publish guard and add release Cursor rule

[T3pp31]

Summary

• Add Cursor rule .cursor/rules/rust-release-cratesio.mdc for Crates.io release / version alignment
• Adjust .gitignore to track .cursor/rules/*.mdc while keeping other .cursor/ local
• Document the publish-guard User-Agent requirement in docs/cratesio-publish-guard-fix.md

Required before merge: release.yml User-Agent

The publish job failed on v1.0.8 with crates.io API 403 because publish_guard uses curl without User-Agent.

Apply this change in .github/workflows/release.yml (line ~128):

-          PUBLISH_STATUS=$(curl --silent --output /dev/null --write-out "%{http_code}" "https://crates.io/api/v1/crates/${{ steps.crate_metadata.outputs.crate_name }}/${{ steps.crate_metadata.outputs.crate_version }}")
+          PUBLISH_STATUS=$(curl --silent --output /dev/null --write-out "%{http_code}" \
+            -H "User-Agent: caesar_cipher_enc_dec-release (github-actions)" \
+            "https://crates.io/api/v1/crates/${{ steps.crate_metadata.outputs.crate_name }}/${{ steps.crate_metadata.outputs.crate_version }}")

You can commit this on GitHub in this PR (“Edit file” on release.yml) or push locally after:

gh auth refresh -h github.com -s workflow
git push origin fix/cratesio-publish-guard-user-agent

(A local commit with this fix already exists on branch fix/cratesio-publish-guard-user-agent but could not be pushed from CI/agent without the workflow OAuth scope.)

Test plan

• release.yml includes User-Agent on publish guard curl
• Merge PR, tag v1.0.9, confirm Release workflow publish succeeds
• https://crates.io/crates/caesar_cipher_enc_dec shows 1.0.9