Bump module path to v3 and add stable exit codes

[AndreyVMarkelov]

Summary

• Migrate Go module path to github.com/dropbox/dbxcli/v3 following major-version conventions
• Add deterministic exit-code mapping (0-8) from JSON error codes for shell/CI scripting
• Add partial_transfer error code for stdout download failures after partial output
• Promote pflag from indirect to direct dependency

Exit code table:

Code	Meaning	JSON error codes
0	Success	—
1	Generic error	command_failed, dropbox_api_error
2	Auth failure	auth_required, auth_refresh_failed, auth_exchange_failed, app_key_required, env_token_still_active
3	Permission denied	permission_denied
4	Not found	not_found
5	Conflict	path_conflict
6	Rate limited	rate_limited
7	Validation error	invalid_arguments, unknown_command, unknown_flag, structured_output_unsupported
8	Partial transfer	partial_transfer

Test plan

• go test ./... passes
• golangci-lint run clean
• go install github.com/dropbox/dbxcli/v3@latest works
• dbxcli ls /nonexistent exits with code 4
• dbxcli --output=json without credentials exits with code 2
• dbxcli badcmd exits with code 7