feat: implement read-only what-if analysis command

[DevAbdullah90]

Description

This PR implements a new /speckit.whatif command that provides a read-only "What-if" analysis for requirement changes. This feature enables teams to preview the downstream impact (complexity, effort, tasks, and risks) of a specification change before committing any changes to the project files.

Key Features:

• Complexity Delta: Calculates a heuristic-based complexity score (1-100) comparing the current state to the simulated post-change state.
• Effort Estimation: Provides a person-hour estimate to implement the proposed change.
• Task Diff: Identifies which tasks would be [ADDED], [REMOVED], or [CHANGED] based on the new requirements.
• Risk Assessment: Highlights potential technical debt, breaking changes, or conflicts with the project's constitution.
• Side-by-Side Preview: Generates a simulated diff preview for the most significantly impacted artifacts.

Testing

• Tested locally with specify version and specify init.
• Verified that the new /speckit.whatif command appears correctly in the Enhancement Commands panel during project setup.
• Verified that command templates (e.g., speckit.whatif.agent.md) are correctly generated in the agent's integration folder.
• Confirmed the execution is strictly read-only and does not modify any local files.

AI Disclosure

• I did use AI assistance (Antigravity) for this contribution.
• Description: The AI helped design the prompt logic for the whatif.md template, updated the Python CLI metadata for command registration, and assisted in revising the documentation to include the new command.

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Fixes #2176

[mnriem]

Please deliver this as an extension as per https://github.com/github/spec-kit/tree/main/extensions

[DevAbdullah90]

I have refactored the implementation to deliver the "what-if" analysis as a standalone extension rather than a core feature.

Changes in this PR:

• Moved to Extension: All "what-if" logic has been moved to a new directory: extensions/whatif/.
• Core Reversion: Reverted changes to core files (README.md, src/specify_cli/__init__.py, and agent integrations) to keep the core codebase clean and modular.
• Extension Manifest: Added extensions/whatif/extension.yml defining the speckit.whatif.analyze command.
• Modular Design: The analysis logic is now contained in extensions/whatif/commands/analyze.md, and I've added a dedicated README.md for the extension.

The feature is now fully optional and can be added via specify extension add whatif. Ready for another look!