fix(orphan): allow orphaning branches whose parent link is invalid

[mvanhorn]

Summary

Fix #116: gh stack orphan now succeeds when the current branch's recorded parent is missing or itself untracked, instead of reporting the branch as "not tracked".

Why this matters

The reporter asked the contributor to evaluate why the orphan check was misfiring, and what the consequences of removing it would be. Tracing through the path:

tree.Build (internal/tree/tree.go:46-54) silently continues when a tracked branch's recorded parent is not itself in the tree map:

parentNode, ok := nodes[parent]
if !ok {
    // Broken parent link - parent doesn't exist
    continue
}

The branch's own node is in nodes[branch], but it is never wired into any parent's Children slice and its own Parent pointer stays nil. tree.FindNode(root, branchName) (internal/tree/tree.go:76-87) walks root.Children recursively, so the disconnected node is unreachable - the function returns nil. runOrphan (cmd/orphan.go:60-63) then translates nil to branch %q is not tracked, which contradicts the git config that still records branch.<name>.stackParent.

The branch IS tracked. The link is just dangling. Treating that as "not tracked" leaves the user with an orphaned config entry the CLI cannot remove, which is the exact symptom #116 reports.

The fix preserves the existing safety check (a branch that has no stackParent config key at all still hits the "not tracked" error) but falls back to cfg.ListTrackedBranches() when FindNode returns nil and the user is allowed to clean up the dangling entry. The children check is safe to skip on this branch: the Build step that disconnected this branch from root also kept it out of any descendant's Parent slot, so a disconnected node never has the kind of children that would silently take an orphan along for the ride.

Testing

• go test ./... is green (the new TestOrphanDisconnectedBranchIsAllowed e2e case lands the regression net).
• golangci-lint run ./cmd/... reports 0 issues.

The new e2e test mirrors the exact scenario from #116: create feat-a off main, then manually rewrite branch.feat-a.stackParent to point at a non-existent branch, then run gh stack orphan feat-a and assert the config entry is cleared. Without the patch the same test fails with branch "feat-a" is not tracked.

Closes #116