docs(readme): sync features, permissions, and roadmap with 0.9.0

[quiet-node]

Summary

• Added /extract, /explain, OCR on text-only models, and KaTeX math rendering to the Features list
• Updated slash command lists throughout to include all current commands
• Fixed first-launch permissions note to mention both Accessibility and Screen Recording (both are required at onboarding)
• Removed stale [model] available config.toml instruction; clarified any Ollama model works via the in-app picker
• Removed "More slash commands" from the roadmap (command set is now comprehensive)
• Removed "image" from the File and document drop roadmap item (image drop already works)

Test plan

• Render the markdown in VS Code preview and verify all sections look correct
• Confirm all slash command names are spelled correctly
• Confirm links to docs/configurations.md and docs/commands.md still resolve