Replace the remaining exec invocations with spawnSync across the VS Code extension codebase for improved security and better argument handling. This change also enables opening shell access for completed tasks in addition to running tasks.

Changes

• packages/extension/src/extension.mts: Replaced async spawn with synchronous spawnSync for git operations and GitHub CLI commands
• packages/extension/src/lib/os.mts: Added new spawnSync wrapper with consistent error handling
• packages/cli/src/lib/git.ts: Updated to use the centralized spawnSync from os.js
• packages/cli/src/lib/os.ts: Exported SpawnSyncReturns type for better type safety
• packages/extension/src/views/: Updated task detail and webview components to allow shell access for completed tasks

Notes

This change eliminates shell injection vulnerabilities by using direct process spawning and provides better error handling. The shell functionality is now available for both running and completed tasks, improving the developer experience when inspecting finished work.

Closes: #97