Modernize Cartero plugin foundation

[mrbrutti]

Summary

• harden plugin discovery with recursive scanning, mixed extension support, and non-blocking warnings
• define plugin manifest schema v1 with capability and trust validation plus fixture coverage
• add a reusable plugin conformance harness with golden output tests for workspace fixtures

Testing

• go test ./...
• go vet ./...
• bash ./scripts/smoke.sh

Related Issues

• Support recursive plugin discovery, mixed extensions, and partial-failure reporting #29
• Define plugin schema v1 with capabilities, trust policy, and manifest validation #30
• Build a plugin conformance harness with fixtures and golden tests #31