feat(cli): add structured exit codes for proper error signaling

[frankentini]

Summary

Closes #1221

Adds an ExitCode enum to garak/exception.py with well-defined integer codes following common Unix conventions, and updates cli.main() to return them instead of using bare exit(1) calls or returning None.

Exit Codes

Code	Name	When
0	SUCCESS	Normal completion (list, version, scan, etc.)
1	RUNTIME_ERROR	General / uncaught errors, GarakException
2	USAGE_ERROR	Invalid CLI arguments or parameter values
3	CONFIG_ERROR	Missing/invalid config files, fixer migrations
4	PLUGIN_ERROR	Missing API keys, bad generator, plugin config
5	INTERRUPTED	User-initiated Ctrl-C / SIGINT

Changes

• garak/exception.py: Added ExitCode(IntEnum) with the six codes above
• garak/cli.py: main() now returns an ExitCode int instead of None; replaced bare exit(1) calls with typed returns; catches specific exception subclasses (APIKeyMissingError, PluginConfigurationError, ConfigFailure) before the generic GarakException handler for more precise exit codes
• garak/__main__.py: Propagates cli.main() return value to sys.exit()
• tests/cli/test_cli_exit_codes.py: Tests for enum values, return types, and exit codes for success paths and various failure scenarios

Motivation

As noted in #1221, proper exit codes help wrapping entities (CI pipelines, orchestration tools, garak-as-a-service) distinguish between different failure modes without parsing stderr output. The codes follow the common Unix convention where 0 = success, 1 = general error, and 2 = usage error.

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[jmartin-tech]

Closing as duplicate of #1651, this can be reopened if that PR does not land.