Add CSV import support for non-TrueNAS sources

- Add truenas_migrate/csv_source.py: parses SMB and NFS share definitions
  from customer-supplied CSV files; returns same dict shape as parse_archive()
  so migrate.py is untouched
- Add smb_shares_template.csv and nfs_shares_template.csv with annotated
  headers and example rows (# comment rows are skipped by the parser)
- Update cli.py: interactive wizard gains a source-type step (archive vs CSV);
  run() resolves CSV source via --smb-csv / --nfs-csv args; --debug-tar is now
  optional; argparse validates mutual exclusion of archive and CSV flags

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

smb_shares_template.csv

@@ -0,0 +1,7 @@
+name,path,comment,purpose,ro,browsable,guestok,abe,hostsallow,hostsdeny,timemachine,enabled
+# Required columns : name  path
+# purpose values   : NO_PRESET  DEFAULT_SHARE  ENHANCED_TIMEMACHINE  MULTI_PROTOCOL_NFS  PRIVATE_DATASETS  WORM_DROPBOX
+# List columns     : hostsallow  hostsdeny  (space-separated; e.g. "192.168.1.10 192.168.1.11")
+# Boolean columns  : ro  browsable  guestok  abe  timemachine  enabled  (true or false)
+# Lines starting with # are ignored.  Delete the example row below and add your shares.
+example-share,/mnt/pool/share,Example share description,NO_PRESET,false,true,false,false,,,false,true