feat: auto-label PRs and issues + sync all repo labels

[Copilot]

Adds automated labeling for PRs (file-path-based) and issues (keyword-based), plus a label definition file and sync workflow to keep repository labels in sync.

Labels (labels.yml)

30+ labels across 7 groups:

• Status: needs-review, in-progress, approved, needs-changes, blocked
• Type: bug, enhancement, feature, documentation, question, discussion
• Challenge: challenge, submission, week-1 – week-4
• Difficulty: beginner, intermediate, advanced
• Contribution: good first issue, help wanted, hacktoberfest
• Resolution: duplicate, invalid, wontfix
• Area: leaderboard, ci/cd, dependencies

PR auto-labeling (labeler.yml + auto-label.yml → label-pr job)

Uses actions/labeler@v5 to map changed file paths to labels on every PR open/sync/reopen:

week-2:
  - changed-files:
    - any-glob-to-any-file:
      - "challenges/week-2/**"
      - "challenges/week-2.md"

Issue auto-labeling (auto-label.yml → label-issue job)

GitHub Script scans issue title + body for keywords and calls addLabels (non-destructive — preserves manually applied labels):

if (/\bbug\b|error|crash|broken|not working/.test(text)) add('bug');
const weekMatch = text.match(/\bweek[-\s]?(\d+)\b/);
if (weekMatch) add(`week-${weekMatch[1]}`);

Label sync (label-sync.yml)

EndBug/label-sync@v2 runs on changes to labels.yml (or workflow_dispatch) to create/update repo labels from the definition file without deleting unrecognized ones.

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