fix(host): group worktree channels under their repository's project

[iamnbutler]

The project picker listed one project per checkout path. Every Codex worktree (~/.codex/worktrees/*/ace2) and Ace lane that had a channel or an opened folder showed up as another ace2 or as the lane name. All of them are the same repository.

• The host resolves each checkout to its repository's main checkout (git worktree list) and its GitHub remote. It caches the result for the life of the process.
• projects returns one project per main checkout, with repo (owner/name). Channel listings carry root and repo, so the app groups worktree channels under that project and puts the project under its GitHub owner in the picker (githubnext › ace2).
• Opening a worktree adds its main checkout.
• Channel records stay as they are. A channel's project is still the checkout it works in (agent cwd and lane base), so existing channels made from worktrees keep working on the same code.

Part of #15. Projects are still keyed per host, so a teammate's checkout of the same repository is still a separate project.

Smoke test: against the canary catalog (29 channels across 10 checkout paths), projects.list() returns a single /Users/natebutler/code/ace2 project with repo: githubnext/ace2. Opening ~/.codex/worktrees/desktop-actions/ace2/apps in a scratch ACE_HOME saves /Users/natebutler/code/ace2. I didn't check the picker UI visually because the dev app couldn't connect: an older dev helper is still running on port 4141.

Built in Ace

Yes. Made entirely in an Ace channel.