refactor: replace cobra subcommands with pflag/viper flags

Replace cobra CLI framework with pflag and viper to use simple flags
(-c for client mode, -s for server mode) instead of subcommands.

This simplifies the command-line interface and removes the dependency
on cobra while maintaining all existing functionality.

Changes:
- Remove connect.go and serve.go subcommand files
- Consolidate all CLI logic into main.go with pflag
- Use -c/--client and -s/--server flags for mode selection
- Update tests to work with new flag-based interface
- Remove cobra dependency, keep pflag and viper
- Update documentation to reflect new architecture

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

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

Author: yuuki

CLAUDE.md

@@ -25,7 +25,7 @@ go test ./...
 
 ## Development Workflow with GitHub
 
-1. Create a new branch for [feature/bug-name] from main branch.
+1. Create a new branch for [feature/bug/refactor-name] from main branch.
 2. Coding the feature/bug.
 3. Add or fix tests.
 4. Check the code quality with testing, linting and formatting.
@@ -37,9 +37,7 @@ go test ./...
 
 The project follows a simple flat structure with all Go files in the top-level directory:
 
-- `main.go`: Entry point defining root Cobra command
-- `serve.go`: Server subcommand implementation
-- `connect.go`: Client subcommand implementation
+- `main.go`: Entry point with flag-based CLI using pflag and viper (-c for client, -s for server)
 - `server.go`: TCP/UDP server logic with context-based cancellation and errgroup for concurrent handling
 - `client.go`: Client implementation with persistent/ephemeral connection modes, rate limiting, and latency measurements
 - `option.go`: Default socket option implementations (non-Linux platforms)
@@ -79,7 +77,8 @@ The project includes test files (connect_test.go, serve_test.go, e2e_test.go). W
 ## Dependencies
 
 Key dependencies include:
-- github.com/spf13/cobra: CLI framework
+- github.com/spf13/pflag: POSIX-style command-line flag parsing
+- github.com/spf13/viper: Configuration management with flag binding
 - github.com/rcrowley/go-metrics: Performance metrics
 - go.uber.org/ratelimit: Rate limiting
 - golang.org/x/sync/errgroup: Concurrent error handling

client.go

@@ -361,7 +361,7 @@ type JSONLinesResult struct {
 func printJSONLinesReport(w io.Writer, addrs []string, mergeResultsEachHost bool) {
 	timestamp := time.Now().UTC().Format(time.RFC3339)
 	results := []JSONLinesResult{}
-	
+
 	if mergeResultsEachHost {
 		ts := getOrRegisterTimer("total.latency", "", mergeResultsEachHost)
 		results = append(results, JSONLinesResult{
@@ -393,7 +393,7 @@ func printJSONLinesReport(w io.Writer, addrs []string, mergeResultsEachHost bool
 			})
 		}
 	}
-	
+
 	for _, result := range results {
 		if err := json.NewEncoder(w).Encode(result); err != nil {
 			slog.Error("Failed to encode JSON result", "error", err)
@@ -412,4 +412,4 @@ func printReport(w io.Writer, addrs []string, mergeResultsEachHost bool) {
 		ts := getOrRegisterTimer("total.latency", addr, mergeResultsEachHost)
 		printStatLine(w, addr, ts)
 	}
-}
+}