Prepare v0.5.9 with plugins directory warning fix

[chaqchase]

Summary

Prepare the v0.5.9 patch release with a focused fix for existing configs whose configured plugins_dir was deleted or points at a missing directory.

Root cause

Config::load validated existing config files before ensuring plugins_dir existed. That made normal listing commands print a warning and fall back to defaults when the configured plugins directory was missing, even though the directory can be safely recreated.

Changes

• Recreate the configured plugins directory during config load before validation.
• Add regression coverage for loading an existing config with a missing plugins directory.
• Promote the changelog entry and workspace versions for v0.5.9.

Validation

• RELEASE_VERSION=0.5.9 .github/scripts/prepare_release.sh
• cargo metadata --format-version 1 >/dev/null
• validate_release_versions 0.5.9
• validate_changelog_section 0.5.9
• cargo fmt --all -- --check
• cargo check --workspace
• cargo test --workspace
• git diff --check
• Smoke test with a temp HOME and missing plugins_dir: no warning emitted and the directory was recreated.