test(self-update): add comprehensive unit tests for self-update command

Add complete test coverage for the refactored self-update command.
Tests validate the full update flow including npm registry integration,
version checking, binary replacement, and error handling.

Test coverage (21 tests):
- Prerequisite checks (2 tests)
  - SEA binary requirement
  - Binary path validation
- Version comparison (3 tests)
  - Skip update when already on latest
  - Update when newer version available
  - Force update with --force flag
- Dry-run mode (2 tests)
  - Check without downloading
  - Verify no file modifications
- Successful update flow (7 tests)
  - Fetch version from npm registry
  - Download package tarball
  - Verify package integrity
  - Extract binary from tarball
  - Create backup of current binary
  - Set executable permissions
  - Clear quarantine on macOS
- Error handling (3 tests)
  - Invalid package metadata
  - Integrity verification failure
  - Rollback on replacement failure
- Stub update (2 tests)
  - Update when launched via bootstrap
  - Skip when not using bootstrap
- Cleanup (2 tests)
  - Clean up after successful update
  - Clean up after failed update

Test approach:
- Mocks all external dependencies (npm-registry, os, env, executable detection)
- Uses real filesystem operations in temp directories
- Validates both success and failure paths
- Tests flag behavior (--dry-run, --force)

Author: jdalton