fix: CSV rows deleted when filtering excludes completed issues

[InDate]

When the issue tracker loads issues with includeCompleted: false (the default), completed issues are filtered out of memory. Subsequent save operations then write only the in-memory issues back to the CSV, permanently deleting completed issues.

Root Cause

• loadIssuesFromCSV() filters out completed issues during load
• saveIssuesToCSV() writes only what's in the issues array
• Any write operation after a filtered load deletes the filtered rows

Solution

• Remove global issues array as state
• Read from CSV on demand with readIssuesFromCSV()
• Use appendIssueToCSV() for new issues (no rewrite)
• Use updateIssueInCSV() for updates (reads all lines, modifies one, writes all back)