Open investigation skills in the project's default-branch worktree

[ooloth]

Why

Investigation skills need codebase access — pressing i on a GitHub issue to understand what's involved requires exploring the actual source, not just fetching API data.

Current state

hub fetch maintains bare clones at ~/.hub/repos/<name>/ but creates no working tree. When i is pressed, launch_ci_investigation opens the skill in hub's own working directory (std::env::current_dir()). A skill investigating an issue in ooloth/dotfiles currently has no direct access to the dotfiles codebase.

Ideal state

• hub fetch creates a linked worktree at ~/.hub/repos/<name>/<default-branch>/ for each project (e.g. main/, trunk/) immediately after cloning
• On each subsequent fetch, the default-branch worktree is reset to current HEAD so it is always clean and up to date
• The default branch name is detected from the bare clone's HEAD symbolic ref — not hardcoded to main
• When i is pressed on any item, the investigation skill opens in ~/.hub/repos/<name>/<default-branch>/ rather than hub's own directory
• The project name is resolved from the item's repo slug via a reverse lookup in Config
• If no worktree exists for the item's project (e.g. the project was added to hub.toml but not yet fetched), the skill falls back to hub's own directory and flashes a warning

Out of scope

• Creating worktrees on-demand at i press — the default-branch worktree is always pre-created by hub fetch
• Changing any skill logic or prompts
• Opening fix worktrees from i — those are created separately when an agent begins a fix

Starting points

• workflows/src/fetch.rs — fetch_project is where the worktree creation and reset belong
• ui/tui/src/main.rs — launch_ci_investigation (line 490) and the i key handler (line 1387) are where the cwd is resolved and passed
• config/src/lib.rs — Config struct with projects: Vec<Project> for the repo-slug-to-name reverse lookup

QA plan

1. Delete ~/.hub/repos/ and run hub fetch — expect each project to have both a bare clone and a <default-branch>/ subdirectory; for ooloth/hub expect ~/.hub/repos/hub/main/
2. Run git -C ~/.hub/repos/hub/main status — expect a clean working tree on the default branch with no uncommitted changes
3. Push a commit to a tracked repo, run hub fetch again — expect the <default-branch>/ worktree to reflect that commit (git log -1 shows the new hash)
4. Open the TUI and press i on a CI failure for a known project — expect the investigation skill to open in ~/.hub/repos/<name>/<default-branch>/ (verify by checking the tmux pane's working directory with pwd)
5. Press i on a CI failure for a project whose slug maps to a known Config entry — expect the skill to open in the correct project's worktree, not hub's directory
6. Temporarily remove a project's worktree directory, press i on an item for that project — expect the skill to open in hub's directory and the status bar to flash a warning

Done when

Pressing i on any TUI item opens the investigation skill in the project's default-branch worktree at ~/.hub/repos/<name>/<default-branch>/.

Depends on

• Local bare clones of hub.toml projects via hub fetch #19 — bare clones at ~/.hub/repos/<name>/ must exist before worktrees can be added to them (now closed/complete)