fix: correct issue metrics and engagement trend calculations

[nehayadav827]

Description

This PR fixes two data correctness issues in src/lib/data-processing.ts related to BuilderOS analytics and engagement intelligence.

Both bugs caused dashboard charts to display misleading values even when valid contributor data was available.

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1. Fix incorrect closed issue detection in processRawIssueMetrics()

The previous implementation attempted to detect closed issues by checking whether GitHub issue URLs contained the word "closed":

entry['Issue Link 1']?.includes('closed')

GitHub issue URLs do not encode issue state, so this condition never worked reliably.

As a result:

• closedIssues was frequently incorrect
• open/closed charts became misleading
• fallback logic relied on a hardcoded percentage approximation

Fix

Closed issue detection now uses contributor-provided issue description fields and checks for natural completion indicators such as:

• closed
• resolved
• merged

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2. Fix engagement trend aggregation in calculateEngagementTrends()

The previous implementation returned hardcoded zero values for:

• High Engagement
• Medium Engagement
• Low Engagement

even when engagement data existed in the dataset.

Fix

The updated implementation now calculates engagement bands dynamically using the same 1 / 2 / 3 participation prefix pattern already used elsewhere in the analytics pipeline.

This ensures weekly engagement trend charts correctly reflect contributor participation levels.

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Changes

• Fix incorrect closed issue detection logic
• Fix engagement trend aggregation logic
• Improve analytics accuracy for dashboard visualizations
• Keep implementation scoped to src/lib/data-processing.ts
• Reuse existing repository data patterns and conventions

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Validation

A standalone local validation script was used during development to verify:

• broken behaviour reproduction
• corrected metric calculations
• engagement aggregation correctness
• chronological ordering
• open + closed = total invariants

Validation output

Ran:

git diff --check

No formatting or whitespace issues were reported.

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Additional Notes

Current repository checks still contain unrelated existing blockers:

• npm run lint opens the interactive Next.js ESLint setup prompt because ESLint is not yet configured.
• npm test -- --runInBand still contains a pre-existing unrelated mismatch in src/lib/__tests__/utils.test.ts.

No breaking changes introduced.

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Checklist

• I have tested this code
• I have linked relevant issue(s)
• I followed the code style of the project
• I reviewed my own code

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Relates to #68

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