feat: add configurable pageSize option

[curtis2point1]

Summary

• Add pageSize config option to customize repos displayed per page
• Default remains 15 for backward compatibility

Problem

The page size (15 repos per page) was hardcoded. Users with many repos or larger screens may want to see more repos at once without paginating.

Solution

Added pageSize field to the config struct:

# ~/.config/git-scope/config.yml
roots:
  - ~/code
editor: code
pageSize: 25

Implementation details:

• New PageSize int field in Config struct with yaml:"pageSize,omitempty" tag
• Default value of 15 set in defaultConfig()
• Validation ensures sensible value (falls back to 15 if ≤ 0)
• NewModel() now reads from cfg.PageSize instead of hardcoded value

Test plan

• Run without pageSize in config — should show 15 repos per page (default)
• Add pageSize: 25 to config — should show 25 repos per page
• Set pageSize: 0 — should fall back to 15 (default)
• Verify pagination controls ([ / ]) still work correctly

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This PR was created by Claude (claude.ai)