Expanded the Rust plugin loader coverage around manifest parsing so invalid
permission values, invalid tool permissions, and multi-error manifests are
validated in a structured way. Added scan-path coverage for installed plugin
directories so both root and packaged manifests are discovered from the install
root, independent of registry entries.
Constraint: Keep plugin loader changes isolated to the plugins crate surface
Rejected: Add a new manifest crate for shared schemas | unnecessary scope for this pass
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: If manifest permissions or tool permission labels expand, update both the enums and validation tests together
Tested: cargo fmt --all; cargo test -p plugins
Not-tested: Cross-crate runtime consumption of any future expanded manifest permission variants
