fix(powershell): treat bracketed feature paths literally

[WOLIKIMCHENG]

Description

PowerShell can treat square brackets in a feature directory name as wildcards, causing task setup or prerequisite checks to miss existing files or match a neighboring directory.

• Use literal-path checks and directory enumeration for feature-derived paths.
• Add regression coverage for bracketed paths and wildcard-shaped neighbors.

Testing

• Focused Python tests: 64 passed.
• Windows PowerShell 5.1: the bracket-path case failed before this change and passed after it; the neighboring-directory case was correctly rejected.
• Sample project: specify, plan, tasks, checklist, and analyze workflows completed with a bracketed feature directory.
• git diff --check passed.

AI Disclosure

• I did use AI assistance (described below)

AI disclosure: AI-assisted tools were used to implement the fix and develop tests.