feat: add URL fetching so Claude can read shared links

[pdsullivan]

Summary

• Adds a fetch_url tool that lets Claude fetch and read webpage content when users share links in chat
• Uses jsdom + @mozilla/readability (Firefox Reader Mode) to parse HTML and extract clean article text
• Adds a tool-use loop to chat() so tools that need results can send content back to Claude for a follow-up response
• Updates the system prompt to tell Claude about the capability and its limitations (JS-heavy pages like X/Twitter may not work)

Test plan

• Send a link to an article/blog post and verify Claude reads and summarizes it
• Send a link to a JS-heavy page (e.g. X/Twitter post) and verify Claude gracefully tells the user it couldn't read it
• Send a non-HTML URL (e.g. a PDF or image link) and verify Claude handles it cleanly
• Verify existing functionality (reactions, effects, image gen, web search) still works after the tool-use loop change
• Test that the 10s timeout works for slow pages

🤖 Generated with Claude Code