fix: keep trailing-slash deobfuscation scoped

[johnson-ricon]

Finding

PR #185 replaces basename with ${arg##*/} for scoped deobfuscation. Unlike basename, that expansion returns an empty string for a path ending in /. Passing notes/<id>/ therefore leaves no non-empty scoped ID, selects full mode, and restores every note instead of only the requested note.

This fix removes trailing slashes before taking the final path component, using Bash 3.2-compatible parameter expansion. The regression proves only the selected note is restored and would fail on PR #185's exact head.

Validation

• Regression on 8c91b5f902943a893b06bd1f48968a77d2478b06: failed as expected because all three notes were restored
• mise run test test/deobfuscate.bats: 30 passed
• mise run test: 415 BATS passed, 9 Python passed
• codebase lint "$PWD": 8 rules passed
• mise run doctor: required checks passed; optional clone-local pre-commit hook is not installed
• readme build --check
• bash -n .mise/tasks/deobfuscate lib/obfuscate.sh
• git diff --check

Fix-it for #185. Base is junior/deobfuscate-path-builtins; this PR does not target main.