feat: Add Hacker News channel (zero-config, public API)

Summary

Add Hacker News as a zero-config channel. HN is one of the strongest signals for tech community sentiment, developer consensus, and early trend detection — and its API requires no authentication.

Why

Agent-Reach covers Twitter, Reddit, YouTube, GitHub, Bilibili, and Xiaohongshu — but Hacker News is a glaring omission for anyone doing tech research, competitive analysis, or developer-focused trend discovery. HN's front page and comment threads are where technical consensus forms in real time.

Proposed Implementation

Use the official Hacker News API (Firebase-based, no key needed) and Algolia HN Search:

• Top/Best/New stories: firebase HN API (no auth)
• Story details + comments: firebase HN API (no auth)
• Full-text search: Algolia HN Search API (no auth)
• User profiles: firebase HN API (no auth)

Tier: 0 (zero-config, zero-auth) — works immediately after install.

Acceptance Criteria

• agent-reach doctor shows Hacker News as ok out of the box
• Channel supports: top/best/new listings, story detail + comment thread, search by keyword, user profile
• Results return structured JSON with title, URL, score, comment count, author, timestamp
• Search uses Algolia API (full-text, sorted by relevance or date)
• SKILL.md documents the new channel with curl examples
• Tests cover: API mocking, URL matching, health check states

Additional Context

• Algolia HN search docs: https://hn.algolia.com/api
• Official API docs: https://github.com/HackerNews/API
• Estimated user value: high — HN is a top-3 signal source for developer tools, startups, and AI trends