docs: SEO and GEO for README, developer guide and llms.txt

[janthoXO]

Makes the repo easier to find in Google and easier for AI assistants (Gemini, ChatGPT, Perplexity, Claude) to cite.

Changes

README.md

• Title with search terms: "automatic audio device switcher for macOS"
• A one-sentence summary under the title: what it does, who it's for, which macOS versions
• Plain section names: Features, System requirements, How to install, How to use, FAQ, Uninstall
• Comparison table: KlangLadder vs. macOS alone. It only compares against macOS, so it makes no claims about other apps.
• FAQ headings written as the questions people search for (AirPods auto-switching, USB mic as default input, device priority, permissions)
• Alternatives section naming AudioPriorityBar, SoundAnchor, AudioWrangler and SoundSource, for "X alternative" searches
• Accuracy fixes: the prebuilt zip is Apple silicon only (built on macos-latest). The Xcode requirement applies only to Homebrew and source builds. Uninstall now points to the right-click menu, where Launch at Login actually lives.

README_DEV.md

• A summary line with the tech stack (Swift, SwiftPM, SwiftUI, AppKit, Core Audio HAL) and more descriptive headings. #raycast-extension is unchanged, so the link from raycast/README.md still works.
• Updated notes that were out of date: the formula is no longer head-only, and the release job already swaps in the path dependency.

LICENSE (new): MIT, the license raycast/package.json already declares. Also added as license "MIT" in the formula (after the url block, so the order still passes brew audit --strict after the release job rewrites the block), plus a License section in the README and a mention in llms.txt.

llms.txt (new, format from llmstxt.org): a summary, key facts and links for AI crawlers and agents.

Outside this PR (repo settings and outreach)

• Done: repo description and 20 topics set with gh repo edit.

• Screenshot or GIF of the popover at the top of the README. Image search and link previews use it.

• Social preview image (Settings → General).

• Submit to awesome-mac and open-source-mac-os-apps. Post to r/macapps and Show HN.

• Later, once notarized: a homebrew-cask entry.

Note: merging to main triggers release.yml (patch bump and release).