refactor: Reorganize flat codebase into package structure

[ravisastryk]

Summary

Restructure the flat file layout into a well-organized Go package structure for better maintainability, testability, and contributor onboarding.

Architecture Changes

Current Structure

etcd-defrag/
├── main.go        # Entry + CLI (mixed)
├── agent.go       # All core logic (~400 lines, mixed concerns)
├── config.go      # Configuration
├── endpoints.go   # Endpoint management
├── evaluator.go   # Rule evaluation
├── disalarm.go    # Auto-disalarm
├── utils.go       # Misc utilities
└── version.go     # Version info

Problems

1. agent.go has mixed concerns - health checks, defrag, compaction, status, orchestration all in one file
2. Hard to test - tightly coupled, can't mock dependencies easily
3. Unclear boundaries - where should new features go?
4. Difficult onboarding - new contributors struggle to navigate

Proposed Structure

etcd-defrag/
├── main.go                              # Minimal entry point
│
├── cmd/
│   └── root.go                          # CLI setup, flag registration, execution
│
├── internal/                            # Private implementation packages
│   ├── agent/
│   │   ├── agent.go                     # Orchestration workflow
│   │   ├── agent_test.go
│   │   ├── compaction.go                # Compaction operations
│   │   ├── defrag.go                    # Defragmentation logic
│   │   ├── disalarm.go                  # Auto-disalarm in agent context
│   │   ├── health.go                    # Health check operations
│   │   └── status.go                    # Member status operations
│   │
│   ├── alarm/
│   │   └── disalarm.go                  # Alarm management utilities
│   │
│   ├── client/
│   │   └── client.go                    # etcd client wrapper & TLS config
│   │
│   ├── config/
│   │   ├── config.go                    # Configuration struct + defaults
│   │   └── validate.go                  # Configuration validation
│   │
│   ├── endpoint/
│   │   ├── endpoints.go                 # Endpoint resolution & filtering
│   │   └── endpoints_test.go
│   │
│   └── eval/
│       ├── evaluator.go                 # Rule expression evaluation
│       ├── evaluator_test.go
│       └── variables.go                 # Evaluation variable definitions
│
└── pkg/                                 # Public API packages
    └── version/
        └── version.go                   # Version information

Backwards Compatibility

No breaking changes:

• Same CLI interface
• Same flags
• Same behavior
• Same output format

Notes

This is a pure refactoring with no functional changes. We should consider adding tests side by side and all existing tests must pass(code coverage expectations can be enforced later if multiple contributors start contributing and we would need high quality bits to be delivered). The goal for this issue is to prepare the codebase for future enhancements like:

• Parallel health checks
• Structured logging
• Metrics endpoint
• Better retry logic
  (These are beyond the scope of this issue, but can be attempted later)

[ravisastryk]

@ahrtr I have opened PR #138 to demonstrate this architectural change/restructuring. Please review when you have time!