feat(app): set up a GitHub project's checkout on this machine before creating channels

[iamnbutler]

Closes #204. Part of #15 (follows #157); dogfooding gap from #5.

On Nate16, ace2 is checked out only on natembp. The project appeared as ace2 · natembp, and none of its channel creation entry points worked. GitHub projects now show their ordinary name everywhere. Folders without a GitHub remote on another host keep the host label.

If a GitHub project has no checkout on this machine, Set up on this Mac opens a setup dialog. So do New channel / + (sidebar, All Projects, dashboard) and GitHub Open in a channel. The dialog has an editable destination (default ~/code/<repo>), and nothing is cloned until you confirm it. The host runs a new local-only project-clone, which does a plain git clone of the validated https://github.com/<owner>/<repo>.git and registers the checkout through project-open.

• Existing folders: a matching checkout is opened instead of cloned. Any other non-empty folder is refused, with the error shown in the form and the path kept.
• Continuing work: creation continues here with its dashboard or GitHub text. Dashboard drafts follow the project's identity, so they survive setup until a channel admits them.
• No gh: gh repo clone isn't used. Its authenticated GraphQL lookup is what organization SAML SSO refused on Nate16, even though ace2 is public. Private repositories use the host's existing Git credentials.

Validation:

• bun types, bun run ci, and the React Doctor diff scan pass. The only warning is the App complexity one that was already there.

• Real smoke through a source ace serve gateway with a temporary HOME/ACE_HOME, no Git config, and no GH_TOKEN:

  • Bad, relative, and unrelated-folder destinations were refused, and the unrelated folder was untouched.
  • ~/code/ace2 cloned public ace2 anonymously and registered as githubnext/ace2.
  • Repeating with GitHubNext/ACE2 opened the existing checkout.
  • create made a channel listed with that repo and root.

• Native, after installing signed Canary 0.0.20 (chore(desktop): prepare canary 0.0.20 #206, build-only run 37650079277, DMG SHA-256 04fe5f0a…1840 matched, Gatekeeper accepted) on natembp and Nate16:

  • On natembp, existing channels and drafts were kept.
  • On Nate16, the sidebar's New channel + in ace2 opened setup with ~/code/ace2. It was edited to /Users/natebutler/code/ace2, and Clone and start channel cloned and created a channel within about 12 seconds, with no SSO prompt or error.
  • The channel (ID a24ec4567772d9e0, renamed in the native UI to nate16-ace2-test) appears with natembp's channels under plain ace2.
  • Read-only peer metadata reports host nate16-1, not hosted, repo githubnext/ace2, and project ID github:githubnext/ace2. Its project and root are /Users/natebutler/Code/ace2: macOS resolved the typed lowercase code to the existing Code folder, so only an exact-string path assertion differed.
  • No model run was checked on Nate16, which has no providers configured.

Built in Ace

Implementation, checks, the smoke test, the issue, and this PR were done in an Ace channel. Outside Ace:

• Coordination and review: Codex coordinated and reviewed the work (Run and resume Codex and Claude Code in Ace channels #9, Wake waiting agents on channel handoffs instead of blocking in shell polling loops #108, Let agents supervise and follow up with subagents #52).
• Remote testing: Codex tested the installed UI on Nate16 over macOS Screen Sharing, because Ace can't operate a teammate host's desktop yet (Operate a teammate host's desktop from an Ace channel #202).