Implement recalculate-start-date workflow to re-anchor all Issue start dates from Project start date

[Copilot]

This PR replaces the recalculate-start-date.yml stub with a full GitHub Actions workflow for project ella-system / project #4. The workflow reads Project start date, computes the offset vs. the earliest Issue start, and shifts all Issue start values accordingly.

• Workflow trigger and configuration

  • Added scheduled execution every 15 minutes plus workflow_dispatch.
  • Configured required env vars:
    • GH_TOKEN: ${{ secrets.PROJECT_PAT }}
    • ORG: ella-system
    • PROJECT_NUMBER: 4
    • PROJECT_START_FIELD_NAME: Project start date
    • START_FIELD_NAME: Issue start

• Data collection (paginated GraphQL)

  • Fetches project metadata and validates required field IDs.
  • Iterates project items with first:100 + cursor pagination.
  • Reads per-item fieldValueByName for both:
    • Project start date (uses first non-null value as reference date)
    • Issue start

• Re-anchor algorithm

  • Finds minimum valid Issue start across all items.
  • Computes OFFSET_DAYS = project_start_date - min(issue_start).
  • Exits early (no-op) when:
    • Project start date is unset
    • no items have valid Issue start
    • OFFSET_DAYS == 0

• Item updates and operational behavior

  • Updates Issue start only when computed value differs from current value.
  • Tracks and logs counters for total, eligible, updated, unchanged, missing, invalid, and error cases.
  • Handles invalid item dates gracefully (warn + skip, without aborting full run).

• Security hardening

  • Added explicit permissions: {} at workflow level.

# Core update pattern used by the workflow
mutation($project:ID!, $item:ID!, $field:ID!, $date:Date!) {
  updateProjectV2ItemFieldValue(input:{
    projectId:$project
    itemId:$item
    fieldId:$field
    value:{ date:$date }
  }) {
    projectV2Item { id }
  }
}