Add generic controller framework inspired by Knative

This PR introduces a new controller package that provides a simple, type-safe
framework for building Kubernetes controllers. The implementation is inspired
by Knative's controller patterns but optimized for simplicity.

Key features:
- Simple reconciler interface with just one method (ReconcileKind)
- Automatic detection and persistence of object changes
- Built-in conflict resolution with retry logic
- Owner reference support for watching owned resources
- Error handling patterns (RequeueAfter, RequeueImmediately, PermanentError)
- Context-aware logging with clog integration
- Full test coverage including e2e tests

The framework handles the complexity of update detection, conflict resolution,
and status updates automatically, allowing users to focus on their business logic.

Also includes:
- Comprehensive documentation in controller/README.md
- Example controller implementation in examples/controller/
- Updates to use errors.Is pattern for error checking
- Minor refactoring to use "if err := ..." idiom where appropriate

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

go.sum

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