Phase 7: Port bbs2gh migrate-repo command

Port the bbs2gh migrate-repo command from C# to Go with full feature parity:
- 5-phase handler: preflight checks, generate archive (BBS export + poll),
  download archive (SSH/SMB), upload archive (Azure/AWS/GitHub storage),
  import archive (start migration + poll)
- Consumer-defined interfaces for all external dependencies (GitHub API,
  BBS API, archive downloader/uploader, filesystem, env provider)
- Two-constructor pattern: newMigrateRepoCmd (testable) + newMigrateRepoCmdLive
  (production wiring with real SSH/SMB downloaders, Azure/AWS uploaders)
- Comprehensive validation matching C# MigrateRepoCommandArgs: source
  mutual exclusion, SSH/SMB bidirectional pairing, AWS credential checks,
  storage conflict detection, archive-path existence, bbs-shared-home check
- 35+ tests ported from C# MigrateRepoCommandHandlerTests and
  MigrateRepoCommandArgsTests
- awsLogAdapter bridges logger.Logger.Info -> awsStorage.ProgressLogger.LogInfo
- Added env.SmbPassword() and filesystem.OpenRead()/DeleteIfExists() helpers
- 0 lint issues (gocyclo fixed via helper extraction)

Author: offbyone

cmd/bbs2gh/main.go

@@ -53,7 +53,7 @@ func newRootCmd() *cobra.Command {
 	rootCmd.Version = version
 
 	// Add commands (will be implemented in phases)
-	// rootCmd.AddCommand(newMigrateRepoCmd())
+	rootCmd.AddCommand(newMigrateRepoCmdLive())
 	// rootCmd.AddCommand(newGenerateScriptCmd())
 	// rootCmd.AddCommand(newInventoryReportCmd())
 	// rootCmd.AddCommand(newMigrateCodeScanningAlertsCmd())