Address type safety and correctness issues from PR #26 review

[Copilot]

Addresses three review comments from PR #26 review thread #3771558129: insufficient API response validation, incorrect mention processing order, and a missing type safety fix confirmation.

Changes:

• Strengthen fetchThread() validation: Add Array.isArray() guard after null check to prevent crashes when X API returns non-array objects (e.g., error shapes). Previous truthiness check allowed objects to reach parseThread() where .length access would throw.

• Fix mention processing order: Process mentions oldest→newest (reverse iteration) instead of newest→oldest. X API returns [newest, ..., oldest], but Set iteration is FIFO. Without reversal, pruning the first N entries deletes the newest mentions instead of the oldest, inverting the intended LRU behavior.

• Confirm type safety fix: Verified parseThread parameter already uses unknown instead of any in index signature (fixed in commit 2407087).

Additional fix:

• Install missing @types/node package to resolve pre-existing TypeScript compilation error.

// Before: Would crash on { error: "..." } response
if (!response || !response.data) return null;
return this.parseThread(response.data);

// After: Validates array shape
if (!response || !response.data) return null;
if (!Array.isArray(response.data)) {
  console.warn('Unexpected response shape from X API (thread): data is not an array');
  return null;
}
return this.parseThread(response.data);

// Before: Processed newest→oldest, pruning deleted newest
for (const mention of newMentions) {
  await this.processMention(mention);
  this.processedMentions.add(mention.post.id);
}

// After: Process oldest→newest, pruning deletes oldest
for (let i = newMentions.length - 1; i >= 0; i--) {
  await this.processMention(newMentions[i]);
  this.processedMentions.add(newMentions[i].post.id);
}

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