[codex] restructure repository documentation

[ThomasK33]

What changed

• Reworked the root README into a clearer product-facing entry point covering what agent-tty is, why it exists, agent-browser inspiration, ghostty-web rendering, quickstart, common usage, limitations, roadmap, local development, docs, and license status.
• Moved detailed material into focused docs for installation, usage, agent skills, and troubleshooting.
• Updated the docs index, contributing guidance, roadmap wording, architecture status, and the dogfood README to match the current agent-tty naming and 0.1.x scope.
• Cleaned a stale live doctor diagnostic string from Agent-terminal to agent-tty and updated its unit tests.

User-facing impact

Humans landing on the repo should get a faster read on what agent-tty is, what value it provides, how to try it, and where to find deeper install/usage/skill/troubleshooting guidance. The CLI doctor --json home-isolation message now uses the current product name.

Validation

• npm run format:check
• npm run test -- test/unit/commands/doctor.test.ts
• npm run lint
• npm run typecheck
• npx tsx src/cli/main.ts doctor --json
• mise run ci