feat: add openspec view dashboard command

[TabishB]

Summary

• Added new openspec view command that displays a comprehensive dashboard
• Shows specs, changes, and progress metrics in a unified view
• Provides at-a-glance project status without running multiple commands

Implementation

• Created ViewCommand class in /src/core/view.ts
• Integrated with existing task-progress and MarkdownParser utilities
• Added visual progress bars and color-coded status indicators
• Archived the change proposal with new cli-view specification

Test plan

• Build project successfully
• Run openspec view command
• Verify dashboard displays correct metrics
• Confirm progress bars show accurate completion percentages
• Check that all existing tests pass
• Validate the archived change proposal

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