feat: Add adaptive diff context size based on selection type

[douglance]

Summary

Implements adaptive diff context sizes that automatically adjust based on the current context:

• Files context: Show maximum context (1000 lines) for thorough code review
• Commits context: Show moderate context (10 lines) for commit history browsing
• Stash context: Show minimal context (5 lines) for quick scanning
• Staging context: Show appropriate context (20 lines) for staging operations
• Patch building: Show extended context (50 lines) for custom patches

Details

The feature is disabled by default to maintain backward compatibility.
Users can enable it via:

git:
  adaptiveContext:
    enabled: true
    files: 1000
    commits: 10
    stash: 5
    staging: 20
    patchBuilding: 50

Resolves #4760

Test plan

• Manual testing of different context views with adaptive context enabled
• Verify backward compatibility with default disabled state
• Test configuration parsing and validation
• Ensure diff context changes appropriately in each view

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