test(cortex-plugins): add comprehensive integration tests for plugin system

- Add integration tests for SDK code generation (manifest, Rust, Cargo.toml, TypeScript)
- Add hook system integration tests (priority ordering, abort behavior, session hooks)
- Add command registry tests (categories, aliases, execution with args)
- Add plugin signing/checksum verification tests
- Add plugin context builder tests
- Add manifest parsing tests with all capabilities, WASM settings, dependencies, and hooks
- Add error handling tests for all error variants
- Add registry integration tests for plugin lifecycle

Total test count: 274 tests (108 unit + 59 edge case + 78 lifecycle + 29 integration)

Author: echobt