fix: address unresolved review feedback — Array.isArray guard, URLSearchParams, type safety

[Copilot]

Implements actionable, unresolved review feedback from PRs #26 and #6.

src/services/xapi.ts

• fetchThread Array.isArray guard: response.data was passed to parseThread() with only a truthiness check. A non-array object from an unexpected API shape would throw inside parseThread when reading .length. Now validates with Array.isArray() and returns null with a warning on mismatch.
• fetchMentions URLSearchParams: Replaced fragile string-concatenated query params with URLSearchParams for proper encoding and readability.

src/services/grok.ts

• Remove any on API response: Replaced const data: any with a typed assertion and added Array.isArray(data.choices) runtime guard, consistent with the xapi.ts pattern.
• Validate parsed action type at runtime: Replaced parsed.action as any with validation against the allowed AgentAction['type'] union, falling back to 'analyze' for unrecognized values.

// Before
const data: any = await response.json();
type: parsed.action as any,

// After
const data = await response.json() as { choices?: Array<{ message?: { content?: string } }> };
if (!Array.isArray(data.choices)) {
  throw new Error('Unexpected Grok API response: missing choices array');
}

const validActions: AgentAction['type'][] = ['reply', 'search', 'generate', 'analyze'];
const actionType = validActions.includes(parsed.action) ? parsed.action : 'analyze';

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[groupthinking]

Closing stale draft — superseded by cleanup/ready-to-use merge to main