fix: resolve CI module linking errors by isolating test mocks from production dependencies

## Root Cause Analysis
CI was failing with 78 "module is already linked" errors due to import chain:
test files → utils/command.js → utils/logger.js → @sentry/node → native modules

This chain caused Vitest's vmThreads pool to load native modules multiple times
across VM contexts, triggering Linux glibc's module linking restrictions.

## Solution: Surgical Mock Function Migration
- Created isolated test-utils/mock-executors.ts with exact copies of mock functions
- Removed all mock functions from utils/command.ts (production dependency chain)
- Updated 54+ test files to import mocks from test-utils/mock-executors.js
- Extracted CommandExecutor and FileSystemExecutor interfaces to separate files
- Maintained zero behavioral changes to preserve all 1046 passing tests

## Key Changes
- NEW: src/test-utils/mock-executors.ts - Isolated test mocks (no production deps)
- NEW: src/utils/CommandExecutor.ts - Command executor interface
- NEW: src/utils/FileSystemExecutor.ts - File system executor interface
- MODIFIED: src/utils/command.ts - Removed mock functions, kept production executors
- MODIFIED: 54+ test files - Updated imports to use isolated test-utils

## Import Chain Resolution
BEFORE: test files → utils/command.js → utils/logger.js → @sentry/node (BROKEN)
AFTER:  test files → test-utils/mock-executors.js (ISOLATED ✅)

## Verification
- All 1046 tests pass locally with surgical migration
- Linting and TypeScript compilation clean
- Production command execution unchanged
- Test mock behavior preserved exactly

Resolves Ubuntu CI "module is already linked" errors while maintaining
test reliability and production functionality.

Author: cameroncooke