feat(charts): visualize pull request activity over time

[marcgc21]

Description

Create a chart to show the number of pull requests opened and merged over time. This helps understand the workflow dynamics, collaboration, and review activity.

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📊 Chart Type

• Line chart or grouped bar chart
• Separate lines/bars for "Opened" and "Merged" PRs

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🧾 Data Requirements

• Source: GitHub API (/repos/:owner/:repo/pulls)
• Fields needed:
  • Pull request creation date
  • Merge date (if applicable)

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✅ Tasks

• Fetch PR data and group by week
• Count PRs opened and merged per week
• Create a line or grouped bar chart
• Add labels for both series (Opened, Merged)
• Include axis labels and a clear legend

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📅 Example Output

X-axis: Week (e.g., 2025-W28)
Y-axis: Number of PRs
Line/Bar 1: Opened
Line/Bar 2: Merged

✅ Acceptance Criteria

• Weekly PR counts shown for both opened and merged
• Opened and merged are clearly distinguishable (color or label)
• Chart is responsive or exportable
• Correctly handles missing data (weeks with zero PRs)