Summary

Adds structured exit codes for different failure categories, enabling programmatic distinction between failure types in CI/CD pipelines and wrapper scripts.

Closes #11

Changes

New: src/cli/exit-codes.ts

• Exports ExitCode const object with 7 named exit codes (0-6)
• Exports ExitCodeValue type for type-safe usage
• Exports getExitCode(err: unknown): number function that maps errors to the correct exit code

The getExitCode() function checks errors in priority order:

1. isAccessDeniedError() → ACCESS_DENIED (4)
2. isExpiredTokenError() → AUTH_EXPIRED (3)
3. isNoCredentialsError() → AUTH_EXPIRED (3)
4. Commander invalid argument errors → INVALID_ARGS (2)
5. isStackInProgressError() / isChangesetInProgressError() → DEPLOY_FAILED (6)
6. Agent not found message patterns → AGENT_NOT_FOUND (5)
7. Default → GENERAL_ERROR (1)

Modified: src/cli/index.ts

• Added import for getExitCode from ./exit-codes.js
• Replaced all 3 hardcoded process.exit(1) calls with process.exit(getExitCode(err))
• Backward compatible: unknown errors still exit with code 1

New: src/cli/__tests__/exit-codes.test.ts

• Comprehensive unit tests covering all exit code mappings
• Tests for access denied, expired tokens, no credentials, commander errors, deploy failures, agent not found, and default general error
• Priority ordering tests to verify correct classification when errors could match multiple categories

Acceptance Criteria

• Exit codes enum/object exported from new file
• getExitCode() correctly maps isAccessDeniedError() → EXIT_ACCESS_DENIED
• getExitCode() correctly maps expired token errors → EXIT_AUTH_EXPIRED
• Top-level error handler uses getExitCode() instead of hardcoded 1
• Unit tests cover all exit code mappings
• Unknown errors default to EXIT_GENERAL_ERROR (1)