feat: support all-in-one request.yaml, enforce gold files, improve README

[zemanlx]

Summary

Support consolidating all test fixtures into a single .request.yaml file, add conflict detection when fields are defined in multiple files, enforce .gold.yaml for mutating tests, and improve README for first-time users.

Changes

All-in-one .request.yaml

• object, oldObject, params, namespaceObject, userInfo, and other fields can now all be defined in a single .request.yaml file
• Alternatively, they can still be split across separate files (.object.yaml, .params.yaml, etc.)
• Golden file (.gold.yaml) lookup works from .request.yaml files for mutating tests

Conflict detection during merge

• mergeTestRequests now returns an error if a field (object, oldObject, params, namespaceObject) is already set when merging from another file
• Removes the old conflictCheck struct and checkConflicts function — conflicts are detected generically at merge time
• Removed redundant auxiliary file loading (loadParamsFile, loadAuthorizerFile) from parseObjectYAML/parseOldObjectYAML — the suite loop already handles these registered file types

.namespaceObject.yaml support

• Added as a new registered file type in isTestFile and testBaseName
• Added parseNamespaceObjectYAML parser with v1/Namespace validation

Enforce .gold.yaml for mutating tests

• If a MutatingAdmissionPolicy produces a mutation but no .gold.yaml is provided, the test fails with: "policy mutated the object but no .gold.yaml file was provided"
• Renamed stale .expected.yaml to .gold.yaml in namespace-selector-binding-mutating tests (was silently ignored before)

README rewrite

• Added Quick Start section with a minimal end-to-end example
• Documented all-in-one vs split file approaches
• Fixed the file type list (added namespaceObject, authorizer, annotations, warnings)
• Fleshed out all four expected outcomes (allow, deny, warn, audit)
• Moved Features to the bottom — usage-first ordering

Test fixtures added

• test-policies-pass: all-in-one .request.yaml tests for validating and mutating
• test-policies-fail: object conflict detection, missing .gold.yaml