Address review feedback: Array validation, chronological pruning, safe iterators, workflow perms

[Copilot]

Addresses 4 review comments from PR #26: inadequate API response validation, incorrect mention pruning order, unsafe iterator pattern, and missing workflow permissions causing CI failures.

Changes

• Array.isArray() guard in fetchThread: Validates response.data is actually an array before calling parseThread(), preventing crashes on unexpected API shapes

• Chronological pruning order: Process mentions oldest→newest (reverse iteration) so Set insertion order matches time order. Since X API returns [newest, ..., oldest], forward iteration caused pruning to delete newest mentions instead of oldest

  // Before: processed newest-first, pruned newest mentions
  for (const mention of newMentions) { ... }
  
  // After: process oldest-first, prune oldest mentions
  for (let i = newMentions.length - 1; i >= 0; i--) { ... }

• Safe iterator pattern: Replace iter.next().value as string with destructured { value, done } check to avoid silent undefined on exhausted iteration

• Workflow permissions: Add pull-requests: write to .github/workflows/pr-checks.yml to fix "Resource not accessible by integration" error when posting validation comments

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